Gragas vs Trundle

Let Barrel Roll (Q) charge before detonating it: damage and slow grow the longer it sits on the ground, and if you don't recast it, it explodes on its own.

Drink Drunken Rage (W) before engaging: drinking takes 1 second, then you take less damage and your next auto hits every nearby enemy.

Engage with Body Slam (E) through a path without minions: you stop on the first enemy you meet, and hitting reduces its cooldown.

After Body Slam (E), cast Explosive Cask (R) behind your target: the blast knocks enemies away from the impact, sending them toward your team.

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.