Can this be used in instances of 'I believe she is great', or would that just be meddwl?
You could use either but, as in English, credu/'believe' has a rather stronger feel to it.
Without more context I would probably tend to meddwl/'think'. It depends on what you are trying to communicate.
Gotta be latinate, this? Are there other words synonymous with 'believe' that may be useful?
As rmcode says, coelio. It is used in the expression:
- Coeliwch neu beidio - Believe it or not