I wouldn't recommand Fallen Corrin actually.
While yes her weapon is powerful and she's colorless, the fact that she needs to be alone is also a detriment to make her a good tank. Having drives and weapons with support effect is essential for a super tank to function. Vantage or not vantage really.
Plus she is still has her dragon weakness. And if she got DC, she will lose her solo skill.
If you want to be cheap like me, use bonus week unit (idealy someone like Fjorm or Legendary Ike) and stack the hell out of them with buffs. Problem solve.