Sion vs Gangplank

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.

Destroy his kegs before he does: a Powder Keg (E) destroyed by an enemy is defused, and each keg loses health over time.

If you can engage, go all-in: his kit has no dash to escape.

If your team has several CCs, don't use them all at once: Remove Scurvy (W) cleanses all his CC, then has a 22 to 14 second cooldown.

In teamfights, get out of his kegs' area: each Powder Keg (E) explosion deals the damage of the attack that set it off, ignores part of your armor and slows you.