Find a water section to fight in, this will prevent the acid from harming you and washes it away immediately!Other than that a good weapon and armor and you will get him!
Other small strategies: a scroll of terror to make him run and than throw some fire at him!
with weapon recommend having a weapon that you have 1 strength higher than requirement, and aim to be level 5 before you meet him.
gaming.stackexchange.com/users/44278/Brent Plante
After playing a lot , here are some tips: The Goo seems to be armored; prefer high-damage weapons to faster but low-damage ones.
The Goo heals 1 hp
every turn in which it stands in water; prefer fighting it when it's on a dry surface.
The Goo's acid washed away instantly in water; so prefer fighting where you stand in water.
When the Goo "pumps itself up", it doesn't do anything for a few rounds, and then does a devastating attack.
The attack has a range of 2 tiles, not just 1 like other melee attacks.
Ways to break it: Leave the attack's range Become invisible Kill
the Goo
So my main strategy is to place myself in water next to a dry place from which the Goo will attack, and when it pumps itself up either stay and hit it (if I have a good armor and HP to spare) or retreat to cancel the attack.
Also, standard boss tricks (becoming invisible and shooting it, using poison / paralysis potions etc.) work well.
gaming.stackexchange.com/users/122/Oak
The easiest solution I've found is a combination of a few of the other techniques mentioned here.
The door to Goo's lair is always 1 wide and usually has some water.
Plant an earthroot seed in the patch of water
nearest the door to get the armor buff and immediately wash off Goo's goo. Plant
any damage over time (fire or poison) seed in the doorway.
Standing on the seed won't trigger it.
Throw something at Goo to get its attention and walk back to your earthroot As Goo approaches throw anything at it for a few hits.
Tank Goo from your watery rooted stronghold.
This approach works with almost any standard armor and weapon combo for any class.
With a warrior I never need a health potion with this strategy.
Wizards are a little squishier but this can be overcome with a potion or two.
gaming.stackexchange.com/users/None/
Don't use daggers.
Use quarterstaff instead.
Make sure you don't let goo touch water though.
It heals 1 health each turn when it is touching water.
If possible, use wands at it like poison and fire.
Throw a potion of toxic gas and block it's entrance of the room.
Just keep on waiting and it will die sooner or later.
gaming.stackexchange.com/users/44371/user44371
How you defeat the Goo is largely up what equipment you have.
I usually hope to find an earthroot and use it to receive hardly any damage, even when he is pumped up.
Use fire material to lure the Goo into a doorway and set it on fire.
Since the door burns it will receive more damage.
gaming.stackexchange.com/users/62035/Raymond
Get the Goo to follow you back to the stairs that go up to level four.
Hit it until it "pumps itself up", or you get its acid on you, or your hit points get too low.
Go up the stairs, wash off any acid and use a portion of healing if necessary.
When you go back down the stairs the Goo is no longer pumped up
and you can safely continue the fight.
gaming.stackexchange.com/users/72566/symanzik
The easiest way is to use the honeypot.
Throw it next to the goo before you Fight.
He helps tank the hits and helps damage.
works every time for me.
gaming.stackexchange.com/users/103737/user103737
The best way to do it that I've found is to stand in water and use a Wand of Lightning.
About six hits with the Wand of Electricity should kill Goo.
gaming.stackexchange.com/users/130308/pixelBoy
Try this tactic.
Plant an earthroot in the door and plant dreamweeds, fireblooms, and sorrowmoss everywhere.
Then (if you have any) throw a potion of poison gas next to goo and then go in the earthroot but DO NOT step off it.
So goo would take damage and if he gets too close you could attack him and soon kill him.
Be sure you have at least the earthroot and poison gas.
Also have a weapon that deals 10 or more damage and armor that absorbs at least 9 damage.
WARNING!
I haven't tried this yet
but it should work
gaming.stackexchange.com/users/144550/Suzzane Bumgardner.
So I got to level 5 of Pixel Dungeon on my Android, equipped with a +3 dagger and a +4 leather armor, and was swiftly defeated by a goo, which kept "pumping itself up" and filling my wounds with acid.  How do I defeat it?
How do I defeat the goo?
gaming.stackexchange.com
<pixel-dungeon>
gaming.stackexchange.com/questions/109059
gaming.stackexchange.com/users/122/Oak
Find a water section to fight in to avoid the acid. Use frie material to lure the Goo into a doorway and set it on fire. The most common strategy to follow, regarding beating the goo, is to fight it inside the water.
The community suggests few tactics to defeat the goo.