Find a door with water on your side. Lure it to you. Plant a fire or poison seed in the door. Toss an earthroot seed into the water. Stand in it. Beat up Goo while it’s being poisoned or on fire.

Make sure you have armour and weapons that arent too heavy. Plant a seed of poison or fire in a door way, piss him off and run.

I mostly play mage, but it should also work when you find a wand or have another ranged class: I just run in circles (entrance room - goo room) and hit goo all the time. When it pumps I run and do ranged attacks until it has caught up or go through a door to surprise attack. This arguably easier with mage because the ranged attack gives 1-2dmg for the next physical attack and 5 shielding. I try to guide it around water if possible.

My only on-floor preparation is to discover both paths to the goo room. I generally don’t go there before I’ve reached 6 talent points (Empowering Meal +2, Lingering Magic +1, Backup Barrier +2, Arcane Vision +1) and dump all my upgrades into the staff, which makes for a strong ranged and melee weapon.

Going through doors probably only works with some kind of mind vision, since you don’t want goo to wander off and heal.

All goo does is: pump slowly (strong attack, but gives enough time to escape), heal on water and give ooze debuff occasionally, but that can be washed away.

Generally don’t get hit by special boss attacks with countdowns (goo pumping; tengu bombs and moving firewall; endboss lasers)