Well, DQ demands you stay around a new surrounding and if you notice you getting defeated easily or having a tough time with boss fights, maybe its best you level up and grind. There's a reason why DQ games involve alot of exploration and patience. You can't just blaze through the whole game barely levelling up. Sooner or later, you'll hit a wall hard and think the game is not fun anymore.
I've found you're better off not trying to grind, as the narrow passageways in the dungeons effectively means a higher encounter rate anyway, and if you get too high a level you can't gain class levels if the monsters are too weak for you.
If someone's struggling on a boss, looking at tactics, equipment and job setups should be the first response, especially with low hp basic jobs like Mage. Seeing as basic abilities carry across all jobs, you're better off switching to a high hp class for the battle if you're having trouble.
Also items- once they are available, have your weakest character carrying the salvation staff (free heals), someone with an Yggdrasil leaf and everyone with a dieamond. They've saved my hide a few times, no reason to go into boss fights with pockets of junk when powerful healing/defensive items exist to help you. What happens is players get lazy and don't tailor item setups to a point they would in other RPGs with a limited inventory for boss fights because the first dozen bosses are very easy.