I'm guessing you're talking about Flanbanero? Flanbanero is resistant to physical attacks, so you need to use magical attacks against him. Air-based attacks are best, but anything other than Fire magic will work.
What happened is it sounds like you may have skipped an area. While it is possible to go to Sunleth directly after Bresha Ruins(I do this every time for the challenge), the level in between is originally intended to be to go to Yaschas Massif 10AF.
Again, it is very possible to do Sunleth before Yaschas, but as far as storyline goes, Yaschas comes before Sunleth, both in actual story and difficulty :D
and the flan are actually like tomatoes :D makes me think of the old cartoon "Attack of the Killer Tomatoes"....oh the nostalgia!