Ah! I see, I blew up that stash by throwing two land mines lol. I knew I was missing something XD Yeah, level design is very hard, specially for puzzles. Just until that level was when I got stuck. If I may add a suggestion, you could do something like this:
Instead of a free fall, make a little turn so mines get stuck in the first turn, but grenades can bounce. Use the inclined surfaces to direct the bounce to the stash, which will be achieved only if the grenade was shot straight downwards. There is still the problem where the player can understand they can actually shot downwards, but this could be an option. Awesome game <3