Senna vs Vel'Koz

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.

Get out of the line of Plasma Fission (Q) and watch its sides too: the bolt splits into two branches at 90 degrees when it hits a target or he recasts it.

Back off after taking two of his spells: at the third Organic Deconstruction (passive) stack, you take a burst of true damage.

If you can engage, go in after Tectonic Disruption (E): it's his only spell to push back someone charging him, and he has no dash to escape.

If you have a stun, a silence or a knock-up, save it for Life Form Disintegration Ray (R): any of them interrupts the ray.