Well, there's plenty of ignorance going on in this thread so I might as well add my own, here are my observations.

Pyro/Geo is actually pretty terrible and it's a choice born out of convenience/ignorance; Pyro consumes Geo and with most spells having a 1AP tax you need Elemental Affinity + The Pawn to pay the tax and you will rarely have a source of Geo that isn't being consumed by Pyro. Plus, too many damn undead so half the school is wasted. So, here's what I end up doing on Tactician:

0 Source: Phoenix Dive -> Flesh Sacrifice -> The Pawn (into Fire) -> Nova -> The Pawn (repositioning into more fire) -> Consider Adrenaline -> Ray -> Consider Medusa ? Medusa Active : Consider Chamelon ? Fireball -> Medusa -> Chamelon : Fireball -> Searing Daggers -> Combust -> Ignite.

Dominate Mind / Teleport can be thrown in there as well.

1 Source: Time Warp -> Flesh Sacrifice -> Medusa / Teleport -> Dive -> Pawn -> Fireball -> Go to 0 Source and start at Nova.

Nova / Ray will generally wipe out magic armor on most targets even if they have resistances. Pre level 20 it's pretty hard for most builds to exceed that output and generally by the end of the turn on the mage you'll have 2-3 targets CC from Medusa's Petrify or whatever else you decide to use.

I would never raw pick Geo as a secondary, I would rather go Summoner before that, but gearing choices will be the factor that swings you in one direction or another -- some times you gotta' go with the bad pick.

It should be blatantly clear that while yes 'Mages' can compete, it takes a lot more effort and pre-planning, and by the end of the game it's all moot anyways as everyone is using Arrow Storm, Meteor, or whatever combined with Shed Skin so they can do it all over again.

The issues with immunity and resistances vary depending on build, but overall I would say it's pretty overstated.

That being said all of Act 2 you'll generally have mages stomp all over the place due to having access to 1-3 Source per fight; chain lightning, summons, and I am sure there are others, tend to shit on everything while melee and ranged have slim to no pickings.