The Flash/Earth 2

With 3 devastating and debilitating passives that shreds all but the most tenacious, Earth 2 Flash can steamroll his foes effectively enough to put even The Flash/Metahuman to shame.

Strategy
Earth 2 Flash essentially has 3 passives: First, he and other Earth 2 teammates are able to stun the opponent upon tag-in, similar to Bane/Luchador's stun, albeit at a lower chance. Essentially unblockable, but not a true block-breaking attack - this stun does no damage of its own and is not a "hit", it simply stuns regardless of block. As such, Superman/Blackest Night's passive does not prevent the stun even while blocking, and it cannot be used to knock out characters, even if they're at 1 health point. This stun will also ignore Invulnerability and will stun the protected enemy regardless. Earth 2 teammates can only stun the opponent on tag-in if The Flash is not knocked out.

Secondly, all of his attacks automatically crit against stunned opponents. This include even DOT effects (even LexCorp Set's 3-piece effect) - but only when he is tagged in. If he sets a DOT and another character tags in and stuns the target, the existing DOT won't crit. DOT set by other characters are unaffected. This makes the Cloak of Destiny pratically mandatory on him as he can take advantage of the bonus damage against stunned as well as the Crit Boost given by the gear. It also gives him a higher chance of stunning the opponent upon tag-in.

Finally, his specials slow his opponent, similar to Killer Frost/Prime's s1 and Killer Frost/Regime's s1 and s2. It appears to stack on one opponent, making them even slower. However, unlike Killer Frost's slow, The Flash's slow can be applied on targets protected by Invulnerability. This is viable against slow characters like Killer Croc/Arkham, who is considered to be the slowest character in the game.

Good with

 * Any Earth 2 characters.
 * Any character that can stun, as that can help Flash if he supposedly fails to stun on tag-in.
 * Hawkgirl/Prime: Her passive allows for quick tag-ins and tag-outs, effectively doubling the amount of times Flash is able to enter the fight and have a chance to stun his opponent.

Good against

 * Characters with slow attack speed, such as Solomon Grundy, Killer Croc and etc.
 * Superman/Blackest Night: As the stun from his passive is not an actual block breaker, it will bypass the unblockable immunity. After which, Flash can deal heavy crit damage while he is stunned.

Countered by

 * Green Lantern/Red Son: He and other Red Son Characters can be immune to both stun and crit, negating the first two effects of The Flash's passive.
 * Batman/Arkham Knight: Though he's still vulnerable to stun, Batman's crit immunity buff greatly reduces the damage he takes.

Abilities
Here are Flash's abilities. Only the very last hit of Speed Vortex can deal a knockout blow out of the whole special.

Support Cards and Gear
Here are Flash's support cards and gear.

Trivia

 * This version of Flash is Jason Garrick from Earth 2 in the comics.
 * Instead of being based on a move, Speed Vortex is based on The Flash's winning outro in the console game.
 * Currently there is glitch where sometimes when the Flash tags in and stuns his opponent the camera will glitch and become static not following the fighters. Tagging Flash out and then back in again may fix the problem.
 * Currently there is a glitch that can make you and your opponent invincible. When you defeat your last opponent and you tag in the flash, his passive could still be triggered. When this happens you see the flash running into the opponent, who lies KO on the ground. But when he hits them, the opponent stands up with 0 health to continue the fight. In this stage, you can only swap characters and perform basic attacks that deal no damage at all.
 * There has also been some instances that during the second proc of the SP2, Speed Vortex, it is possible to knockout an opponent, but the exact way to do it is currently unknown, but very possible as some players can do it. But one theory suggests that the opponent must be the last opponent to beat and he/she must be suffering from DOT and have less than 5% health.