Hmm... It MIGHT be your browser? some browsers don't act right with GDevelop web player? Otherwise, the way to do it is 1-beat mushroom boss, then the lizard boss. after beating lizard, go right one screen, then up. Just hold space till you jump over the bridge (it will auto jump over the bridge if you hold space), then head left till you get the the other water area. If for some reason you CANT leave the first waters 'hallway', then I have no idea how to help.
Like I said at the top, maybe try a different browser. I play tested (AKA beat the whole game) on Windows10 with Chrome. (1660ti GPU, 10500 4.5Gh CPU), so the only thing I can suggest is the browser. sorry.
Haha, funny thing: This time I tried to play in chrome, got to the lizrd as usual and spammed fire.
got his health down to zero (or below) and then that guy jsut regenerated both health bars.
attacking him further (element doesnt matter) shows damage done animation but health bars dont decrease.
and he keeps doing his water and tongue attacks as usual.
seems like spamming him to death with fire has turned him into an immortal god instead :-D
so sadly I cant tell you if that other thing also happens on chrome.
as I cant get past the lizard anymore :-/