Specifically the reason Angela gives for holding back her power is "if I did not maintain the charade of normal human strength, the Divine Order would throw THEIR full strength at thy capture," which is true for long enough to make it to the confrontation with those two angels before the final boss. Past that, though, the party's just very tough themselves!
If you'd like a little bit more lore about the nature of the Divine Order, it comes up again in The Fate of Another World... and if we ever did get a chance to continue the game's story further from there, what it means for the Divine Order to throw its full strength at you is definitely near the top of the list of questions I'd love to answer.