Given that you have 3 physical damage characters, I would recommend just going with a full physical damage party. It doesn't matter how much physical armor a mob/monster has if 4 characters are focusing to go through it.
Doing this, Abao's tankmage is a fantastic option and would be a perfect fit for your party. The Warfare/Geo suggestion is more of a base fighter build and works well. A lone point into Pyro can be useful to light all of the oil on fire if you need to do some magical damage. This is good for sword and board Battlemages. Because you already have Red Prince set up as a fighter, there are a couple other good options that will both fit in well with the party.
Alternatively, you can still go more of a traditional Battlemage by doing a strength build with a focus on Warfare and just a couple points in Aero. This allows access to Uncanny Evasion and with a point in Scoudrel will open up Smoke Cover as well. These 2 spells are useful for a 2 handed Battlemage or Dual Wielding Battlemage. Dual Wielding get more interesting later into act 2 as you can max out Warfare, and start dumping to Dual Wielding to get your Dodge up. Choose gear wisely and a dwarf (Beast in this case) can get Dodging up above a 30% chance with little effort. Add in those 2 defensive Aero spells and your Battlemage gets pretty difficult to even hit let alone take down.
Yet another, less traditional option for your Battlemage would be an intelligence/Necro build. With high intelligence Decaying Touch and Mosquito Swarm do LOTS of physical damage. As you will be burning through Physical Armor you still want a couple points into Warfare. This will allow not only the CCs from Battle Stomp and Battering Ram but also Bouncing Shield and Deflective Barrier to do some more physical damage. This build will require sword and board set up. Also, a point in Hydro and/or Huntsman is useful to get the most out of Decaying Touch when it does get past armor. Hydro also opens up Raining Blood synergies with Blood Sucker. Interestingly a point in Pyro works well here. This allows crafting of Corpse Explosion (Necro/Pyro), which does good physical damage. Yes, the combat system is versatile enough that even an intelligence build can be a physical damage power house. You won't be relying on auto attacks, so the wand that does the most damage will be your choice.
All three of the above builds are melee, physical damage builds. As such, they would round out your group well as a full physical damage party, which is a solid group set up.
Abao's Pyro Battlemage is a fantastic build which can be tons of fun, but is doing magical damage while the rest of your party is doing physical and as such may never burn through the magical armor of opponents to do actual damage to their health bars. That would leave this type of Battlemage really only doing CC with Battle Stomp and Battering Ram after the rest of the party has stripped off physical armor. So, while overall it is a fantastic build, it likely won't fit your party make up. This build is ideal for either a full magic damage party or a mixed damage party. A mix party would have 2 magic damage and 2 physical. I would highly recommend doing another play through when your current one is done and making a party that would make the most of the Pyro Battlemage.
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