Sion vs Trundle

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.

Don't fight in Frozen Domain (W): he gains move speed, attack speed and increased healing inside. Back off to draw him out of the zone.

If you deal physical damage, back off after his Chomp (Q): it steals some of your attack damage for a few seconds, and he gains it.

Get away from Pillar of Ice (E) as fast as you can: it slows nearby enemies and blocks the path while it stands.

Back off when he casts Subjugate (R) on you: he steals part of your armor and magic resist, and gains what you lose.