[Coda]

That's why I ran King slimes when I was at your level -- giving up a sub slot in exchange for that kind of spike damage against a high-defense boss is usually a worthwhile exchange.

Another good choice is poison (Cleopatra and Lilith are the easiest to get).

Defense breakers are hard to make useful. Top Droidragon and non-Awoken Ifrit are worthwhile but both are hard to get, and Machine Golem, Dark Golem, and Awoken Ifrit only do 75% defense reduction (and are also hard to get) so you still have to have some pretty hefty spike damage on the team. Depending on the boss, this could be more or less useful than a 3xATK bonus from a King slime. The Barons are only 50% defense reduction and are almost certainly not worth it.

If the boss only has double-digit HP, another option is to stack tons of HP and RCV (I've been known to run 4/1/4 or 2/2/4) and just ping through the dungeon. It's slow but effective.

I can give more specific advice if you tell me which dungeon(s) you're stuck on.

EDIT: One more point in favor of King slimes -- they work against bosses with status ailment resistance, while defense break and poison obviously do not.