Acrid deals huge amount of damage after stacking poisons from his Festering Wounds, Caustic Sludge, and Epidemic. Try to stun targets inside your Caustic Sludge for maximum damage. Remember that you can fight at melee and range!
Caustic Sludge has a 17 second Cooldown, but the trail itself will last for nearly 15 seconds, meaning that by the time it dissipates, Acrid will be ready to spread another trail shortly. Spam it as often as possible, as it slows enemies down and speeds allies up, making it great not only for combat, but for escaping as well.
Try to spread Caustic Sludge in as wide an area as possible, avoiding bumpy, uneven terrain as much as possible.
Hanging on a rope after spreading Caustic Sludge is a safe way to kill a big group of enemies, as this will keep them grouped and most of them can not reach the character.
The Caustic Sludge can easily take out a group of imps holding a reward, as you can spray the ground directly next to the statue, summon them, and they'll die easily and quickly, granting you an easy and painless reward.
Epidemic will spread on contact with other mobs, as well as attempt to jump to others close enough on the screen. It can multiply itself and do devastating damage to groups, and can go through walls. Note that it won't return to a previously infected enemy.
As a class that must move around in the fray, and works well with hit-and-run strategies, items such as the Red Whip, Paul's Goat Hoof, Hopoo Feather, Photon Jetpack, and Rusty Jetpack can help him quite a lot. Items that increase movement speed can help him spread Caustic Sludge much further, and jetpacks and hopoo feathers can allow you to repeatedly hop above a placed area of sludge to lure enemies into it without putting yourself in harm's way.