In ZELDAThe Four Swords Adventure ,What kind of projectile weapon kills the monsters in the lake in Level2?

Answer 1

the hookshot (which isn't available for a long time) can either kill them or stun them and bring them close to you

where there are shallows, your sword can reach them

the bow and arrow, which also isn't available until after you beat "level 3" as you call it

don't worry about killing them - their primary reason for being there is not so you can kill them, it's so that when you get the flippers and swim, you'll have a hard time. when you are swimming, you're not able to use any weapons in this game.

the boomerang stuns them (i don't remember where you get the boomerang, or when, but it's possible you don't have it yet)

the fire rod does a little damage (not available yet)
the ice rod freezes them, (not availble yet) and makes them freeze and sink forever below the water, but you get no items for it

each of the three magic discs (not available yet) can kill them
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Answer 2

Use a slingshot, or throw bombs at them when they surface.