The game is certainly not easy. You need to optimize carefully your build or you'll get beaten very often in the mid/endgame.
I've played A1-A3 and they were much easier, you could use 4 mage-priests and blast your way through anything on torment, not here. Also magic and prayer skills do NOT raise your mana! This was a huge difference with the original trilogy (dont know about A4, A5, havent played them)
Warriors suck until the end of the game where you get some super weapons (pole) that make them mean.
You most consistent source of damage are mages. Also use the mage summon spells heavily.
I use one warrior-meatshield-priest, 2 mages, 1 full time priest, worked fine to beat the game on medium, but it wasn't a breeze. This is playing to win reasonably fast, doing some sidequests but focusing mostly on the main ones.
Also hang on to your scrolls-wands-potions! Some of these are essential for winning. Most important are energy potions and scrolls of speed (actually increase your action points unlike the haste spell). Mass madness is great in the middle game.
I used scribbane 4-5 times in the hard fights (works similar to a scroll of speed), pretty useful replacement for the much rarer scrolls of speed.
Don't use it more than 5 times, after just 4-5 uses its benefits are very short (hastes you for only a turn or two). Plus you risk getting the withdrawal symptoms (not very bad anyway).
In group battles focus on keeping people alive.
Anyway this is by far the best Avernum of all time, HUGE and very diverse, great quests, challenging, a wonderful game!