RTF is a global modification that replaces all towns in the game.

The onyx elementals simply turned out to be very weak compared to the lava elementals, and in terms of stats, the lava elementals are better and have more useful abilities.

Indeed! It has become quite weak; the calculator shows that 18 hit points and infinite retaliation are equal to magic immunity, fire immunity, and a bunch of minor stat boosts!!! I suggest simply giving the elemental 75% magic immunity, then it will become competitive, and give the lava elemental a single point of initiative.