There are a few types of battlemages. If you have trouble doing much damage with your battlemage, you're probably in category 1 or 4.

-The most useless battlemage ever:
You want to be able to deal both physical damage and magic damage? This is the kind that decided that splitting points between physical damage skills/attributes and magical is a good idea. 2 mixed "battlemage" like this is half as good as a pair of mage+warrior, in terms of total damage dealt; even if you find a situation in which only, say, physical damage is needed, having a focused physical damage dealer do everything the 2 of your mixed "battlemage" do while the focused mage skips their turn is slightly better than having 2 mixed "battlemage", as the focused physical damage dealer at least does the damage in a single turn instead of across 2 turns.

-The not really a battlemage:
Went pure intelligence with no warfare, holds a stave, spams spells, and only use the stave for when you ran out of cooldowns. It's a powerful build, it's easy to use, but while you might fancy yourself a battle mage, you're really just a normal mage.

-The tankmage
Build like a normal strength warrior, with a few points in geomancer for buffs and crowd controls (fortify, mend armor, earthquake, earth spikes, reactive armor). Mainly build warfare and one handed, do only physical damage and negligible earth damage, spam knockdowns slows and cripples, and basically be a nuisance to your enemy. Don't expect to actually do a lot of damage tho.



-The actually good battlemage:
2~3 points in warfare, the rest in two-handed and pyrokinetic, and focus the entire build around sparking swings. Equip a fire stave, proc loads of sparks with each whirlwind/onslaught/battering ram/battle stomp/blitz attack/crippling blow, and deal mostly fire damage. You'll be a mostly fire damage dealer, otherwise you won't do much damage. If you play it right, over half of your damage will be from the sparks. This is one of the builds that does the most damage in the game until lvl 16 where 3 source skills break the balance.

-The not really good battlemage:
Exact same as the actually good battlemage except you use a stave that's not fire damage and/or not use sparking swings. And yes, going venom coating lands you in this category too. For other staves, the problem is that you won't be able to just dump points in pyrokinetic for damage increase across the board, and splitting points between 2 elements is just weakening both. As for not going sparking swings... you'll basically be a normal knight that does magic damage. While that may not sound bad, most enemies don't have physical resistance, but most have some form of elemental resistances, so you deal a lot less damage than a normal knight. And you won't be able to knockdown enemies because you don't break physical armor.


P.S. Sparking swing only crits if you have savage sortilege.