Otherwise, max all your buffs - Bless, Heroism, Shield, Regen, Protection from Water etc - and just beat on him. There is a theory that real-time combat is better than turn-based as the opponent's reaction to the previous blow can create a window for the next character to slip ahead of the "action clock" and get the next hit in, whereas in TB, the opponent will always get their turn in the proper order. So keep that Attack key pressed in RT and if one of your characters goes down, drop into TB to heal until they're back up, then go back to RT.

If this doesn't work, try upping your skills for a few levels - especially to increase some crucial buffs from Basic to Expert or Master - and come back.

One of my favourite things about this game is that, early on, your melee is super weak, as is your overall party, and your buffs only apply to a single character, so they're inefficient to apply, and generally not worth it. So you start by dropping into TB for every battle and carefully picking your way through your best option every turn (spell, attack, heal etc). Then, once you get to a certain level of Spirit/Body Magic (and eventually Light, if you're on that path), and your buffs start applying to the entire party, and lasting longer, your ability to survive increases exponentially, and melee becomes a more realistic option. But if you've already trained yourself that magic and TB is the only way to survive, you might not ever think to change. It's honestly my favourite phase of the game.