How do I beat the giant nougat monster?

The easiest way is to get the rocket boots from the desert fortress. Equip the obsidian crown and the monkey staff. Then just hold j to stay in the air until your fireballs kill him. No sweat

To beat him I used the octopus king crown with obsidian, the summoning tribal spear, and the boots of introspection. Just wait while you spawn minions. When there's enough minions between you and the monster, or when the monster wakes up, start shooting fireballs (having the red shark fin also helps).

Get your magic upgraded at well using pain au chocolat a couple of time and then go attack him with no weapon equipped. Put thorns shield everywhere and keep teleporting when you are near him, when you have put enough thorn shield wake him up with a fire ball and then teleport back he will go thru all the thorn shield and die.