If Oath is like the other games in the series, the auto-revive is a one-shot that you should save for Galbalan anyway. (I've never actually burned the auto-revive early and seen if it became repurchasable, though.)
Your experience sounds like mine - run the dungeon, get walled by the boss, collect 1-2 levels, boss is now beatable. Except. As noted above, I have utterly cheaty weapons for Nightmare that would take a ton of extra grinding (if even allowed? On my Normal playthrough, they capped the items at+3 upgrades) to get, which means I'm probably stealthily 2 levels higher functionally than I seem, which since level is the god stat, well. So congratulations on your awesomeness if you're not doing crazy hour-long grinding per boss.
Elf: Oh of course, great characters can save objectively stupid plots all the time (cough, cough, Game Arts. Or for that matter several Fire Emblems). So yeah, good thing for XF the villain cast hams it up well, I'm down for hyping that part.