Any tips on getting past Bjorn the Behemoose!.
I am level 27 and am fighting with sabrecat (lvl 24), rotten apple (max lvl 20) and slime knight (lvl 24. Is there a good strategy or do I have to just do some serious grinding?

Insulatle doesn't do anything to protect against lightning.

Read the description of the spell. Fire and ice.

I used insulate and i definatly reduced damage. Still having problems getting past bjorn coz of his double attacks. So gonna do some grinding

make rotten apple use sap. This will reduce Bjorns defense a lot and attack with the hero, saber and slime knight. If he does use the lighting attack heal with slime knight or the main character. Using sabers war cry would also speed up the process(i did about 312 to Bjorn after sap was cast)

You might want a spellcaster with high defense. Powerful spells would be able to cause massive damage to bjorn, while also providing necessary support spells.

-Pre-Battle
Make everyone LV.30+
Give everyone at least 1 Yggdrasil leaf
Give hero the best single target weapon you have
-Battle-round 1
Have the boy cast insulate & kabuff first
Have the girl Oomph the hero
-Rest of the battle
Have the hero attack constantly & heal then needed.
Have the boy heal and slip in an attack when you can
Have the girl cast Kasap
-Side notes
Fill the spare slot with a monster that can attack or heal constantly. use it to the best of it's abilites

You'll definitely want Hero and Parry. A Golem would also be a good idea (very easy to get and very strong against physicals) mine was laughing off anything that wasn't a Desperate Attack).