The dragons can be killed in any order you want. Two of them are in the final dungeon, so if you've got 2 left before going there, you're still good to go.

I would say don't level up. Not unless you get game overs repeatedly. These dragons are supposed to be challenging, but managing to kill one means you're still strong enough. The Wind Dragon isn't the easiest of them either.

The next time you play through the game, I'm sure you'll find the dragons easy even if you are 5 levels lower than you are now. Last time I played I think i was in the late 30s when I killed the last boss.