Gangplank vs Garen

Build a chain with Powder Keg (E): detonate the keg farthest from your opponent, and each keg whose area touches the previous one explodes too, all the way to them.

Last-hit minions and pop kegs with Parrrley (Q): a kill with the shot gives bonus gold, and so does a kill from a keg it sets off.

Save Remove Scurvy (W) for a key CC rather than for healing: it cleanses all CC on you, on a 22 to 14 second cooldown.

Cast Cannon Barrage (R) anywhere on the map to help a fight or cover an objective: each wave of cannonballs slows and damages in the area.

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.