Senna vs Alistar

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.

Take a trade right after his Headbutt (W): without it, he can't engage from range, and its cooldown is 14 to 10 seconds.

Back off when his carry is close enough to follow him, and play hard when they're far away: an Alistar engage only kills if someone follows up.

Once he hits level 2, stay out of Headbutt (W) range: combined with Pulverize (Q), he knocks you into the air as soon as he arrives.

If you have CC, save it for after Unbreakable Will (R): the spell removes the CC on him when cast, but doesn't protect him from what comes next.