It will be some assembly but it should hopefully not be too complicated -- you are editing within game parameters rather than creating whole new code sections.
Anyway you lose some life as a result of this sort of thing. Find the life location in memory (infinite life cheat will have it, or you could just remake such a cheat).
Now you have the life location set a breakpoint on it (I think we are still suggesting bgb debugger for GB/GBC debugging so bpw aka break on write). Go find some lava, water, ledges and sinkholes (make a custom level if you want, savestates should also be an option) and jump off/into them. The game should then write to the health counter and the debugger will say hold up something wrote to it. Look back up the instructions preceding this and something should be a subtract somewhere along the line. Change this from probably 2 or 4 (Zelda health counters are hearts and if they count in quarters then you have 4 life per heart) to something that will kill even a maxxed out player and you have your hack (I don't recall a lives system or bottle fairies in this one).
I imagine the reset part at least will be common between the various hazards but do check. Alternatively you could find the thing that happens when the falling/drowning/whatever occurs and branch immediately to the death event but that would be a bit harder if you are not used to assembly and the major damage thing is a perfectly fine of doing the hack as well (I occasionally do some dubious stuff to achieve results but that is not one of them).