Scar

Scar is a very difficult Hero to play who thrives based off whether he can manage to land his instant kill ultimate or not. No character is so completely defined by this ability, as his damage output is generally very poor without instantly killing targets. Enemies have many ways to deny his ultimate kill ability, so he will have to be skilled in order to land it, or have to rely on his allies to help him do so.

Abilities
D - Vengeance: Scar will gain +25 to all stats the first time he kills a State Alchemist. If Scar joins the Alchemists, he will lose this ability and not be able to gain any more stats. If Scar kills a State Alchemist other than a boss, he will be unable to join the Alchemists. If Scar joins anyone whatsoever, his kills for killing State Alchemists will downgrade to 15 per Alchemist. Edward, Mustang, Alex Armstrong, Marcoh, Kimblee, Isaac, Basque Grand, Tucker, and Giolo count as State Alchemists. Players on the Alchemist team can also voluntarily become State Alchemists, at which point you will be notified.

Q - Deconstruction Alchemy: Scar uses his right hand to destroy the ground in front of him. This deals 300/365/430/495/560 damage and a 1/1.14/1.26/1.38/1.5 second stun to anything in the path of the deconstruction. The ground is destroyed in a 600 range line that is 300 range wide. 600 cast range, 35 second cooldown. Costs 75/100/125/150/175 Mana. W - Into Smoke: Scar uses his alchemy to destroy the ground underneath him and shoot up debris and smoke all around him. This deals 300/350/400/450/500 damage to nearby enemies in a 300 area of effect and make him invisible for 7/9/11/13/15 seconds, or until he attacks or uses an ability. While Scar is invisible, he gains 100 movement speed and is surrounded by smoke that makes it obvious to enemies where he is. 29 second cooldown. Costs 130 Mana. E - Ishvalan Monk: Scar was once an Ishvalan Monk and as such he has been heavily trained in hand to hand combat. This gives him a 10% chance to critically strike for 2/2.5/3/3.5/4X damage and 7/13/19/25/31% Evasion.
 * This ability's cooldown is reduced by 3 seconds per upgrade.
 * This ability deals 0.5X Scar's Strength in bonus damage per upgrade.

R - Destruction's Right Hand: Scar uses the deconstruction process of Alchemy to shred through his foe. This will cause each strike Scar deals on a target to deal more damage than the previous for 10 seconds. The second strike deals 2x damage, the third 2.5x damage, the fourth 4x damage, and so on. This bonus can stack with Critical Strikes. If Scar hits a target 7 times in 10 seconds, they will die. This will not instantly kill bosses and has no effect on NPCs. Scar only needs 6 hits if he's not on the Alchemist team. This also boosts Scar's movement speed by 10/20/30% and his attack speed by 33/66/100%. 85/70/55 second cooldown.
 * If Scar uses Hatchet while he uses this ability, the target affected by his R will be chosen randomly from the targets he damaged. Don't use hatchet with R.
 * The counter on enemies is stored directly onto the unit, so transformations, even those that aren't immune to it like Roa and Envy's shapeshift, will reset the counter.

Playing as Scar
Scar is all about landing his instant kill ultimate. He can do some damage to NPCs if left alone with them thanks to his passive, but as far as enemy players, his damage will strictly be his ultimate. He can get some damage on enemies if he's neutral with his upgraded W, but by the time he will have enough upgrades for it to matter, it will be close to promised day, where Scar will almost inevitably lose. Scar has no realistic hope of surviving Promised Day, even with allies. His ultimate does nothing to Father, so he will generally lose to him alone even if all homunculi and alchemists are dead.

Scar's most important goal that makes him able to function is the ability to stun people for his ultimate. His Q is his only stun within his kit, and that will never be enough against a competent homunculus. Scar absolutely needs to get Terraforming Alchemy Book, and Great Maul/Bald will also greatly help him with stunning targets to death. What he needs above anything else, though, is allies to help stun him for the ultimate. None Scar will have his ruins casually backdoored while he's out, and have a severe lack of stuns to help him do anything. Neutrals Scar still will almost always lose his base, but can at least have another Hero help to stun for his ultimate, such as Jerso or Darius.

Scar's only other positive trait aside from the ability to instantly kill people is his high health and his W enabling him to escape. If a teamfight goes on long enough, circling around like a shark waiting for the kill with R will greatly help you. Enemies will generally be content to ignore Scar, so stacking items on him is a good idea to force them to target him. Items, most prominently Bald, will greatly help him in landing his ultimate. If Scar's enemies are willing to chase him despite how slippery he is because of him carrying valuable items, he may get the opportunity for a surprise kill on them.

Not joining the Alchemists is extremely ill advised as Scar, but if you must, you want to have an extensive amount of State Alchemists on the enemy team, along with stupid Alchemist players who bought Silver Pocketwatches. Scar will need all of the stats to have any hope whatsoever of winning, and it is still completely impossible to do under any realistic circumstances unless he has at least one neutral ally to help him.

Make sure to try to kill Isaac and Basque before joining the Alchemists, as you will still get some stats for them if you kill them while neutral. Basque has a very tight window to be killed while joining the Alchemist team, as Scar can join alchemists between 36 and 33 minutes remaining on the Promised Day timer, while Basque spawns at 35 minutes remaining. Make sure your team helps you with getting kill credit on Kimblee and Tucker if they exist, as Kimblee will typically always deny himself while Tucker will be too well defended behind his traps and Nina.

You need to be extremely aware of all of the abilities that counter Scar's ultimate when playing him, and to wait until they are used or bait them out. Forcing a hero to use their ability that blocks it without your R, or having your allies do so, will give you a prime opportunity to use your R once that ability has expired. You will also want to use R shortly after a hero uses their jumps so they can't just casually run away. If all else fails and you're a very weak Scar on the Alchemist team who can't seem to land any ultimates, you can use your R exclusively to bait enemy abilities to waste them. Let your team carry you, while you go in first to soak damage because that's all you're good for.

Playing against Scar
You have absolutely no reason to fear Scar other than his ultimate. His damage output is very poor outside of his instant kill ultimate. He is somewhat tanky and has the ability to escape, but once his ultimate is used, he has already made his teamfight contribution. Scar should be targeted last in a teamfight if his ultimate has been used. Alphonse, while much harder to kill than Scar, contributes far more to a team fight than Scar could ever hope to outside of his ultimate. The only reason to give more priority to Scar is to get important items he's carrying off his corpse.

Sloth's D Sleep, Lust's D Charm, Wrath's R Ultimate Eye, Jerso's W Noxious Gas, Izumi's W Sand, Barry's R Butcher Spree, and the R transformations of Gluttony, Envy, Cornello, and Greed all are invulnerable to Scar's ultimate. Alphonse W is the only spell that is surprisingly vulnerable to it of these types of abilities. All of these are very easy to use to counter Scar directly.

Kimblee can just play at a range to avoid Scar given how Scar is strictly melee range, and can deny himself from being killed with his R. Tucker can make himself immune with E tentacles, and use Nina to taunt Scar to force him to waste his R. Pride's R will occupy Scar for too long to let him use the ability, and can easily escape with his multitude of copy stuns or the tank. Olivier can hide in her tank to escape Scar's ultimate. Envy can always just rely on his ultimate to avoid Scar's ultimate, but can also shapeshift to reset the counter of Scar's ultimate, which will usually be enough to stall him. Because shapeshift is typed and not a pushable button, this can be done even when Envy is stunned and is a casual get out of jail free card for him.

Scar absolutely needs to combo his target down by making use of Terraforming Alchemy Book, his Q, and Great Maul. It is absolutely worth popping your designated ability that counters Scar R early if the Scar is actually threatening enough to do this, either by having lots of nearby allies, items, or stats. Once you have stalled his R, Scar is worthless and can do nothing but sit around and tank.

A Scar who has high enough stats to be threatening to a team is probably neutral, and as such has his very vulnerable Ruins of Ishval in the corner of the map which he can't defend. The Ruins of Ishval are the easiest neutral base to backdoor due to how far away they are from everything else, making them impossible to defend. You can then enslave Scar and force him onto your team by using a gate. Fu, Lan Fan, Ling Yao, and Wrath are best for backdooring the ruins and not suffering any penalty for this. If the Homunculi do not have Wrath, waiting until Promised Day is a fine solution, as Scar will never get an ultimate off while he is in the Father Room.

Scar is the #1 user of Terraforming Alchemy Book in the game due to the synergy with his ultimate, but Terraforming Alchemy Book does not stun the target, it only roots them in place. All abilities are still useable while rooted, so you can use your designated ability that blocks the ultimate with no issue. If that ability is on cooldown, a jump ability will usually suffice to stall long enough to block the ultimate, or even casting a stun/knockback on Scar like Gluttony W. There is no shortage of ways to block or stall out Scar's ultimate, nearly every ability with stun, knockback, or mobility can prevent it easily.