The strategy that I've used for this fight has been pretty consistent:
Round 1: Aura uses MATK All, Charlotte uses Concentrate, John uses Counter, Paul uses Health All
Until Arwin is dead: Aura uses thunderbolt or fire on Arwin, Charlotte uses fire on Arwin (or concentrate if she lost the buff), John uses protect, Paul attacks Mammon
Afterwards, do the same exact thing while hitting Mammon instead. Aura should use light now and Arwin's slime should be killed last.