Sion vs Gwen

Last-hit minions and monsters for Soul Furnace (W): each kill permanently gives you max health, more on champions, large minions and large monsters.

Slow your target with Roar of the Slayer (E) before charging Decimating Smash (Q): they're less likely to get out of the area.

Charge Decimating Smash (Q) for at least 1 second: below that, the hit only slows, and beyond it, the knock-up and stun last longer the more you charge.

After you die, hit the nearest champion: during Glory in Death (passive), your autos are much faster, heal you and deal damage based on their max health.

Cast Unstoppable Onslaught (R) from far away to engage or roam: unstoppable for 8 seconds, you stun longer and hit harder after a long charge.

To hit her during Hallowed Mist (W), walk into the mist or wait for it to fade: from outside, you can't target her.

Take short trades before she lands 4 autos: each one adds a snip to Snip Snip! (Q).

Step out of the Needlework (R) line: she can recast it twice within 6 seconds, with three then five needles.

Answer her split push with two players rather than alone: A Thousand Cuts (passive) heals her on every auto against a champion.