I'm not an Etrian Odyssey expert by any means, but I'd say you just need to grind. Find the classes with the skills you enjoy, make sure your party has variety, such as including a protector, damager, healer, elementalist, and buffer/debuffer, and then just grind levels. Etrian Odyssey games are notoriously difficult, especially when playing on the hardest difficulty setting. Don't try switching classes too mcuh, though as in most games you lose levels and skill points whenever you do. Just keep grinding and it'll eventually get better.