You could make something that checks when a unit enters a region (the region being the area maybe, 50 units around your unit, you'll have to adjust that until you get it right) it kills your frog.
Or you could make it a more niteresting version of frogger and give all the enemies a boosted immolation, and then instead of lives, you get one frog (no hp regen) and you can get hit a few tiems on the way across (maybe 100 hp, with an immolation that drops 50/sec, just over 2 hits kills)