The Giant Slime is the 3rd boss in the game, and is located in Brightstone, inside the 'The Oozing Dark' stage.
Acid Spit - Launches a blob of acid, which becomes a puddle on the floor. Units that stand on this take constant damage until they leave the puddle. The puddle disappears after a moderate length of time.
Transform - The boss splits into 6 small slimes when he loses a certain amount of health. The slimes will come back together to reform the Giant Slime.
This boss fight is fairly simple, however if your team does not have a high damage output, it can be time consuming. The Acid Spit ability is usually a non-issue, unless you do not notice that an important unit is standing upon it. Otherwise, simply move your unit. It is a good idea to use 2 Ranged classes, the Cleric, and a Rogue, with the rogue acting as the 'tank', as the mages/witch/ranger can move and still attack unimpeded. Kill as many of the slimes as you can each time before they get back together, and it will help to speed up fight. You can also use a move that can make the enemies fall to get a chance to attack the slime.