Man. Mech Terra's Limit Break magic based or what? It hits for less than a Vivi auto attack.
And my best unit right now is King Giott lol
Yes, her limit break is magic based, as is all her specials outside of Metal Kick.
As someone who is using her until I roll something better, her problem is that she appears to be intentionally gimped to be just short of greatness. Her maxed 4* form only has a base MAG of 71, which is well behind Vivi's 92.
For spells to really start doing meaningful damage, MAG really needs to be at least 150 and higher. The vast majority of solid mage gear right now is locked behind INT Colosseum, which is going to require characters considerably stronger her to obtain. Even with all that gear plus Shiva, she might start to approach 150 MAG. That said, the difference from +21 MAG in Vivi's favor is substantial, as attack ratings are exponential in terms of damage dealt.
She's strong enough at max to clear the 2nd isle, but I have significant doubts about her being strong enough to clear the 3rd isle when it releases. Vivi should last considerable longer than her, and even more so if he ever receives a 5* upgrade.