I completed the game, including the bonus monsters and the Menacing Monsters Quest, on normal, without using any Summoned Armor (an idea suggested by Azyleo). So then I tackled the game on Hell using almost the same tactics, and never any Summoned Armor. I again defeated all monsters, including the "secret" dungeon.
I did use the extra character, but I don't think her help was important. She is so weak (low level), I wasted turns healing her. If I had had the brains to include the priest healing skills in a second character (I only had 1), I doubt the extra character would have been needed. As some of the posters have noted, the "Ninja" class skills do help (Cocoon insured I would have somebody survive a big attack), although I did not keep a Ninja class character after mastering the class.
Oh I loved the extra character! (To be honest though I just thought the whole concept of unlocking an additional party member was cool, and didn't ever pay that close attention to how strong/weak they are compared to the rest of the party.)
Playing this game on Normal could be a disappointment for anyone who wants the battles to be at all challenging. 8-O But I found the overall story to be more compelling than any of the boss battles, so I wouldn't hold that against the devs in this case.
@1oldtymer, suggestions don't need to be followed so as long as you can beat a boss without using specific skills you're good
And something Off-topic. For unknown reason I can't change the text from Bold to normal (whenever I hit that button to remove it from next text, it still appears again as soon as I try to type.. Strange. )