May I ask why you would do that?
If you can consistently hit two body parts with dynamites, 17 of them (23 in expert). Worst case scenario, 149 grenades hitting the mouth (208 in expert).
EDIT: unless you mean explosive blocks, in which case 6 assuming you hit all 3 body parts (8 in expert) or 17 assuming you only hit the mouth (23 in expert). I would go for the average (12 in normal, 16 in expert)
I'm in desperate need of entering hardmode on my latest run.
If there's no reason to do it with explosives. Then you're better off with using normal weapons.
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