I knew that @Tarbosaurus will post his suggestion here!
@DinoMan to see this, check here: https://metahub.info/jwa-dinosaur/majundasuchus/
I think we cannot ignore that, with counter attack plus the weakest attack, a lev 18 Majungasuco can give 1218 damage… just to compare, for example, with a brand new lev 16 Stegodeo with her armor piercing rampage 1266 dmg but cooldown 1 turn.
Plus swapping a dino for your Majungasuco cannot give to your opponent a free hit, seen her counter attack, thus Maju is a good option, until creating new and / or stronger legendaries.
This is the right question, and it can be answered just by you If you don’t mind to “waste” coins on a creature that won’t be used in the upper arena, or you don’t have in the near future a stronger creature to level up, then…
But, in your shoes, I would keep fusing for 20 coins each, to gain experience to go to your lev 12 and not evolving her to lev 18.
Hope this helps to think about it and take the right decision on your style of play.