I'm not so sure about that. Warrior seems like a subclass meant for people who would be staying in most of the time. And well, Flandre is kind of a hit and run character. Although she has a butt load of HP, I really don't believe she'll be able to utilize Warrior to her full potential. She benefits a lot more from Gambler.
Unfortunately, that would mean that the "Vampiric Wrath" skill would be rendered useless. And I have to ask at this point: why does Flandre, of all people, have a skill like that? I could think of a much better skill for her, truth be told. I forget what the name of it is, but Yuugi had a skill that increased her attack power the less health she had, so why can't Flandre have something like that? That would have some synergy with her "Starbow Break" spell, since she can lower her own health and thus increase her strength with each strike. It would be risky, but it would make more sense than "Vampiric Wrath" does now(Hell, you could call the "Less HP = More Damage to Enemy" skill the name "Vampiric Wrath" when it's on Flandre, and it would make perfect sense).
Monk is more of a stay in subclass yeah, but Warrior also seems to be for those who stay in too.
Personally, I would think that Monk is for those who want have more flexibility in whether said character is to stay in or will swap out, given its emphasis on speed and accuracy. You can go either way with it.
Quick question: how many bosses are resistant to Cold element spells, as compared to the other elements that said bosses could possibly resist? Because since Cold element spells are the hardest to get in the game(other than the few characters who have it naturally, you can only get such a thing from the Toxicologist subclass(one of the reasons why I sing more than a few praises about it)), so...