Sion vs Garen

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.

Hit him regularly, even with a small poke: Perseverance (passive) only regenerates him if he hasn't taken damage recently.

If you're ranged, kite him: his kit has no dash, and only Decisive Strike (Q) speeds him up and removes his slows.

Wait for the Courage (W) shield to end before your burst: the spell starts with a shield and tenacity, then keeps only a damage reduction.

Once he hits level 6, back instead of staying in lane at low health against him: Demacian Justice (R) deals true damage that grows with your missing health.