Trundle vs Gragas

Start your trades with Chomp (Q): the empowered auto slows your target, then steals some of their attack damage, which matters most against a physical damage dealer.

Take your trades near the wave: King's Tribute (passive) heals you for part of the max health of each enemy that dies near you, minions included.

Place Frozen Domain (W) where the fight will happen and draw the enemy into it: inside, you gain move speed, attack speed and increased healing.

Place Pillar of Ice (E) behind the fleeing enemy: it blocks their path and slows nearby enemies.

Cast Subjugate (R) on the enemy tank: you steal part of their armor and magic resist, and your team can focus them down.

Stay behind your minions: Body Slam (E) stops on the first enemy it meets, minions included.

Get out of the Barrel Roll (Q) area right away: the longer the barrel sits on the ground, the more its explosion hurts and slows.

Step up when Body Slam (E) hits nobody: its cooldown isn't reduced and stays at 14 to 12 seconds.

Spread out from your allies: Explosive Cask (R) knocks every enemy away from the impact, which can split you up or send you toward his team.