Angelo learns multi-heal somewhere around level 28-30. Its pretty much a must have when fighting dhoulmagus, because if you dont that means the hero has to heal along with angelo every round. Dhoulmagus has alot of powerful group attacks and without multi-heal your offence is pretty much cut in half.

Get the boomerang sold at the bazzar in arcadia, pick up some silver mail and combine it with two dragons scales for some dragon mail for yangus. If you beat level C at the monstor collosium, you'll get saint ashes, and mixing that with the zombie mail found on dhoulmagus's island will give you some REALLY buff armor for the hero. Also mixing two dragon scales with a steel shield will give you a dragon shield, more buff armor. Also mix two dragon scales with a snakeskin whip to make a dragon tail, too give jessica a bit more... Oomph in her offence. If you've been buffing up her whip stats, she should have a two hit whip attack that when equipped with a dragon tail should deal around 100 points of damage all told. Go too the southern desert, there is an inn there where you can pick up a powerful sword for angelo, that will put enemies to sleep. Plus in that desert there are three very powerful monsters you can recruit for your team at the monstor collossium, including a bone rider who can attack twice per round, and has around 500 hp.

Other then that, just level up. Thing about dragon quest is you gotta take your time and move slowly, beat up on monsters and always make sure you have the most powerful equipment you can. In Final Fantasy its a luxury, in DQ it's a neccessity to be as powerful as you can. What I do is whenever Im waiting for the alchemy pot to do its thing, I'll do a tour of duty in whatever place has the most powerful monsters, that way Im sure to get experiencea and gold. Also hang out at trodain castle, there is a certain breed of metel slime there that will give you 10,000 (nope, thats not a typo) xp if you defeat it.