Thanks to the new armor system, mixed parties are less effective. Pure physical parties with support mages are probably the strongest, at least in the early phase. Later mages could get stronger, I'm not that far to really argue about that.
There is not synergy between Str, Fin and Int because they have no secondary effects, so splitting up will weaken your damage in the end.
Fire and poison (earth) has the strongest combo, because of the explosions. Water and Air are low on damage an offer mostly CC, which is useless as long the enemie has still magic armor left.
Oil is probably one of the strongest debuffs, because it ignores armor.
Necro/Hydro-Healer is less a healer, more a close-range-mage, because of decaying touch. Necro does not really offer heal except selfheal, but if the healer needs healing something went wrong, I guess. Necro is more usefull for tanks probably.
Metamorph is something, you can probably take on everyone, because it offers a lot of utility.
If you play undead, you need worry about fire a lot and enemies tend to go an heal you instantly.