What's a good tactic to defeat Giant Piranhas?

Piranha rooms are a form of special room, and somewhere on the same level will be a potion of invisibility. This will allow you to sneak past and get to the chest they are guarding.

However, this is a badge for killing 6 piranhas, and sneaking past doesn't count. So , provided you have an item you don't mind potentially losing propping open the door (if the strategy fails you'd be hard pressed to get it back) the following will work (in order of fun and wickedness):

• Wand of poison - it will take a while, but will work... eventually.
• Potion of toxic gas - you'll need a few, but they will die quicker than poisoning each fish.
• Wand of lightning - only works if they are bunched up as the lighting only jumps to adjacent blocks. If you stand in the door they will rush you, step back and they will all be near each other
• Wand of teleportation - a classic, this teleports them away, however you don't seem to get XP for the kills, but they will die and count to the badge. You also risk teleporting them to water elsewhere in the level and running into them again.
• Wand of amok - this is an uncommon wand that turns creatures against each other. Zap each piranhas and they will attack other creatures - but creatures don't seem to retaliate. Since you can inspect creatures with the ? icon, make sure the last creature is always the one with the least health and then attack as normal.

What doesn't work:

• Wand of Firebolt - the damage of time aspect goes away
• Potions - freezing, fire, and paralysis are all useless.
• Don't try and fight them, they seem to each get 2 hits for your one. So 3 pirahnas get 6 hits to your one, and they are tough.

They are too tough to fight effectively. When confronted with piranhas, I do one of two things:

1) Potion of Invisibility - This is the "official" solution to these rooms. If a piranha room is spawned, a potion of invisibility will also spawn on the same floor. Drink it at the door and walk around them, grab the treasure and leave.

2) Wand of Lightning - if you zap a water room with lightning, it will damage all creatures inside. A couple zaps should do it.

Again, don't bother fighting them. Your HP is better spent fighting other monsters.

Be the huntress, throw some inert object (food, meat, blandfruit, stylus) in the door and kill 'em with the boomerang. I actually consider this a built-in bonus of the huntress -- extra food whenever you find piranhas, and extra invisibility potions you don't have to use to get past pirhanas.

Shattered Pixel Dungeon now gives a range of 2 to the spear, so in that variant spear-fishing is an option.