Mages are weak? Yea, right! I started off playing a backstab character (till Act 3)...until I started playing around with pyromancy. I do agree that in starting off as a mage (especially as lone wolf) can be challenging, but as the game progresses you just get stronger and stronger.
Using just necromancery and pyromancery I lone wolfed the entire game on classic mode. Once my build hit its peak (around act 2), I pretty much started 1 shotting everything. The most important thing for me besides mobility, was making sure I went first. So I made sure to toss a couple points into wits, especially after maxing int.
My tooltip damage for spells like fireball was about 3-4k, though my crit chance with it was about 65% (Savage Sortilege ftw). So most of my spells hit for about 5-6k, and the big ones crit could crit as high as 12-18k (avg, have crit even higher). The most insane part was most spells were either AOE or a cone. Keep in mind fire also does burning damage on top of all of that....
My personal combo for starting fights was flaming skin + peace of mind + supernova. If you can line yourself up to hit as many around you as possible, this combo is golden. Normally I had enough AP left over even after it to do another spell like laser ray or firewhip (depending on the situation).
If I was using this on a boss, I would use gap close (phoenix dive or cloak and dagger) + flameskin + peace of mind + adrenaline + supernova + haste + flame tongues (if anything survived or melee was still on the battlefield) + chameleon cloak. Keep in mind I probably opened with laser ray.
So with this setup if any enemy survived and tried to walk out of the fire I put all around me, they would get hit by flame tongue (which hits super hard lol). Otherwise, I would sit there for 2 turns building up AP and waiting on cooldowns.
Since I was playing lone wolf on classic, my build was using sword/board (happened to have a sword with +25% crit bonus and some nice stats). To do more damage I could have dual wielded weapons for higher crit, but I got to addicted to shields up for harder fights.
I also suggest always using necromancery as a backup to deal with anything "immuned" to your primary spell damage type. Since you still get the scaling bonus from your int for necromancery, it can still do decent damage. Its also handy at lower lvl when some mages have extremely high magic armor, but low physical armor.
There are some spells that can be casted from stealth that are great openers for combat, and others that will get interrupted before you finish (due to dialogue). I also agree that from my testing so far, pyromancery spells seem to do the most damage out of all of the other elements.
I will have to do more testing on this, as the stats on my of my gear late game was all pyromancery. In fact that's yet another insane part of mage, I wore boots through the entire game that had 20 physical defense on them. I used them purely because they were a unique that gave +1 to pyromancery. I was looking for a replacement...but never found one lol.
If you choose to use supernova...keep in mind without flameskin you will need to have more magic armor than the damage you deal with supernova. Otherwise its possible to 1 shot yourself