I don't think you can get away with slugging it out with groups toe-to-toe until your well above L30 and have some good armour, some significant health regen buffs going and you're dealing 1000+ damage per hit. Until then you have to fight clever using specific tactics for different situations.
A typical way i would deal with a tight mixed group would be:
rush the most dangerous target
whirlwind the group
firewall the group
thousand strikes the most dangerous target
Maybe one more hit on most dangerous target if thousand strikes almost got him/her
jump out or roll away. Opportunity for ranged magic and recovery.
repeat to finish off target or target next most dangerous when rush and whirlwind cool down
I use pause to do it systematically and quickly - take a lot less damage that way. The whirlwinds and firewalls don't kill outright but they do significant mass damage.
I found it's not too good to have a high level rush 'cos it tends to knock the target over which is more of nuisance than anything if they are your prime target. Magic blast is very usefull against groups of rangers/mages/summoners because they will try to run away to hit you from range leaving you swinging at air and taking damage and magic blast seems to stop them in their tracks for a short time enabling you to get a couple of good chops in on them.
Very few enemies are able to survive rush + thousand strikes, but IMO you want your points in thousand strikes, not rush, so they are still standing after you rush them.
Finally, do you have summon demon and have you got your creature up and running? They make a big difference if you let them take the aggro first.
