I was bored so i started a new file in this game. This game is very short and would only take you 2 hours or so (if you are planning on getting all of the S-Flags)
The Sub-Boss, Dragon Fiends, have a hell load of power. If you get caught in one of their grapples, say goodbye to about 1/2 your health.
Tentacle Fiend on the other hand deals a lot of damage too, but it is easier to dodge. As long as you keep your distance and attack when necessary, you'll do just fine.