There isn't exactly a strategy that we can post.
We don't know what type of weapons you chose or which ones you bought or found. We also don't know what kind of armor/accessories you have. If you have them, try to equip items that give high protection but also prevent status ailments that can keep you from fighting like Madness, Stones, Paralysis etc.
The best thing I can tell you is what RPGeez said. Go to Grom's skill list and make sure that Enemy Knowledge is turned on. Click on Enemy Knowledge and when it asks you if you want to see the messages tell it yes.
After the fight begins, check to see what it says the monster is weak to. If your Enemy Knowledge spell is strong enough you will also see what the monster is immune to. Avoid those things. Then exit (use F12) and see if you have any weapons that are that weakness. For example, if it says he is weak to fire, see if you have any weapons that are fire based weapons. Don't equip a very weak weapon to get this though. Then see which of your characters have spells that are also that kind of spell and use those.
Cast spells like Grom's Protection and Rayanne's Boost to see if that helps. Keep your people alive. If that requires that you fight a few more rounds so that you can cast heal/aura spells so be it. Pay attention to you mana points as well as your hit points. It doesn't help if you run out of mana and can't heal/protect you characters.
Pay attention to your fight. If you are fighting and two of your characters are getting good hits and the other two are getting mediocre hits or hits that heal the monster, use two for fighting and two for healing/protection.
I hope this helps.
dee0227