It doesn’t look overpowered to me - at 3rd level it just amounts to a +1 bonus to Dex, Con, attack, and damage, and +2 DR. You don’t get the second +1 until you reach 5th level, and the next +1 at 10th level, and so on every 5 levels. That is, when the rules say things like “per 5 levels”, you need to have all 5 levels before you get the additional bonus, as opposed to “5 levels or portion thereof”. Also, since it specifies caster levels, that means that if you take a level of, say, fighter when you hit 5th level, you don’t get that next +1 until you take another level of cleric.
A barbarian gets DR 2 at 10th level, not exactly “high-level”, and the barbarian’s DR is “always on”, whereas the DR from Battle Mastery is a temporary effect (5 rounds + Cha modifier). And I suspect the full text of Battle Mastery indicates some kind of limit on how often the ability can be used, as I would agree with it being overpowered if you were able to keep it up all the time. When cleric domains grant abilities like this, they’re usually limited to “once per day”, making them essentially “emergency” abilities.
I couldn’t find anything at all when I Googled “Battle Mastery”, so I decided to search on the full text of the ability. Here’s what I came up with:
All of the links I found that matched this appear to be connected to the Neverwinter Nights 2 video game (which is based on D&D/Forgotten Realms), as opposed to the tabletop version of D&D. So I’m going to make a guess that the War domain ability was changed for the video game for one reason or another. I’ll speculate that the reason is that dealing with the whole “deity’s favored weapon” thing was determined to not work well for the purposes of the video game, so they changed it. Alternatively, the video game may be based on the 3.0 rules, rather than the 3.5 rules you’re playing with, and I’m afraid I’m unfamiliar with the particulars of 3.0. Is it possible you were looking at a 3.0 Player’s Handbook when you found that ability? I also see you used an online character creator; it’s entirely possible that tool was outdated.
Aha, I found this on the Wikipedia page for Neverwinter Nights:
So that would appear to be your problem - you found the ability in the 3.0 version of the rules, rather than the 3.5 version. There were, I understand, many significant rule revisions between versions 3.0 and 3.5.
So yeah, if you’re playing by 3.5 rules, then your War domain ability is simply Proficiency and Weapon Focus with your deity’s favored weapon.