And I found the answer : just a sphere trigger bound to a regen health of the boss.

The tricky part is that I have to add these logical schemes to every boss room.

Adding the logical schemes isn't really tricky once you know how to do it.
Searching the names of the boss room layouts though, that's something else.

The particular case would be for bosses having several phases, since each phase is technically speaking a different monster. Namely, the Djinn, the Dark Alchemist and the Netherlord.

But it would be a bit too tedious to have to go through all the phases again..
I'm thinking especially of the Dark Alchemist battle. Every battle is a pain in the ass, with huge framerate drops due to to the flames on the ground. I wouldn't want to start it all again everytime I die. And if I wouldn't want that, I can imagine how the usual player would feel :P

So I guess that's not a bad thing that it would not restart the phases.

I might have to rebalance some of the boss fights (in terms of damage), to tone down the one shotting abilities. Indeed, having a one shot ability would be hardly manageable with such a modification.

But let's do it iteratively.
First, apply the change to every boss. Then I'll worry about the damage.
Players can always switch to Veteran or Normal if they are stuck and frustrated anyway.

I have spent quite some time figuring out where did the Boss actually spawn.
First, I had to find which map to modify, and then I had to find out how did the game communicate which boss to spawns in this map (because a same map can be reused for Mapworks or some other fights, and yet have the same layout).

Also, the teleporter mechanism is a bit counter intuitive ; you would expect to have a "teleporter" object, to which you bind a "destination point". But actually, you just have a "teleporter" which acts as an actual "teleporting point", and you have to trigger it with a player sphere (or some other stuff if you want !). Once it's triggered, no matter where the player is, it is teleported at the right place (the place of the teleporter).

What is interesting is that you don't necessarily need to be in a particular place to be teleported.
You could build your logic of the map in such a way that once a monster is dead, you get teleported. Or you get teleported once you open a chest.

Which could lead to interesting traps to do, and I'm actually quite surprised that the devs never thought of it. Anyway, keeping it in a corner of my head.. That's for sure an interesting mechanism to explore.

On a side note, I really hope the other boss fights don't have randomized rooms.
It's just painful to have to do the same modification 4 times, and with different sets up (since the room is different).

Also, what makes me spend a bit of time is the teleporter stuff.
Or how to block a path in a neat way.

If you ever encounter a giant shovel blocking your path, that's because I was out of inspiration.

Aside from the giant spider, I can't think of any other boss room that is randomized.
I hope to finish the first Act soon !