I found a way to make spis easier.

EXPLANATION: When you will throw last pirate into the target, switch immediately into Asterix which is stuck in the monkey skull, then move almost into the middle of the monkey skull, and then mash the menhir rain combo. When combo will start, and you will switch to Obelix, try to walk as close as possible to the volcano (hold up right direction) and spam switch button. Looks like if both characters are closer to each other, switch happens a lot faster. After switch to Asterix which after menhir rain is placed on the ceiling of the monkey head, still hold up right direction, so he will turn back, which will prevent him from falling down of the collision.