Senna vs Braum

Attack the souls held by the Mist whenever you can: each soul freed with Absolution (passive) increases your AD, attack range and crit chance.

Fire Piercing Darkness (Q) through an ally or an enemy to hit both: the beam heals your allies, and damages and slows enemies in the line.

Cast Last Embrace (W) on a target that's already slowed or immobilized: after a short delay, they and nearby enemies are rooted.

Cast Curse of the Black Mist (E) to engage or escape with your team: allies who enter it are camouflaged, then fast and unselectable.

Cast Dawning Shadow (R) to save an ally across the map: the beam has unlimited range and shields allies.

Back off when you have Concussive Blows (passive) stacks on you: his allies' autos add stacks too, and the 4th stuns you.

Catch him when he's far from his allies: Stand Behind Me (W) only leaps to an ally, minions included, and without one nearby he has no dash.

Don't shoot into Unbreakable (E): it intercepts your projectiles. Go around it, use AoE spells, or wait for him to lower his shield.

Don't line up with your allies in front of him when he has Glacial Fissure (R): it hits every enemy along its line and around him with a knock-up.