1: (ab)use Freaks: The good mutations can make soldiers that are far superior to those of other nations. The promotions to be most aware of are
Heroic Attack I, Heroic Defense I, Strong, Heavy, Light, Mobility I, Immune to Disease. The weak freaks can be used to create entertainment in your cities (Freak Shows)
2: Loki is situational: Loki is a special "hero"; He's available right off the bat, but he's not a combat hero, and he's not capable of levelling. This means he basically has two uses; in peace, he can convert enemy cities with no culture (this is a good way of barring in civs - especially those who tend not to be strong enough to punish you. Remember the diplomatic penalty from using this ability), or he can serve as an early caster of Chaos I (1 FS chance). Always consider if you can spend the

better on other things.
3: Make the most of your traits (obviously): This is nation-specific because, well, as Balseraph you will sometimes be playing as Perpentach, the only leader whose traits are changing. Especially using the "half cost of..."-attributes of the different traits is smart when you have a lot of improvements from your cities to chose from.
4: The Balseraph lean towards early war: Mostly due to the Freak mutations, but also because they (unlike a number of other civs) do not have a 'favourite' path you can venture down, tech-wise; No need to rush for a religion or for arcane techs. Instead, you can kill off those who would try this and become a problem further down the line.
5: The Arena's special ability is mostly useless: Your armies will
not benefit from this kind of training. The mortality rate is far too high for Arena fights.
6: If possible, get your hands on a unit with Medic II early (priests): This will allow them to remove diseases and witherings, making a far wider percentage of mutated units viable soldiers.
7: Mimics are fun, but they need support: You need to compensate for the initial weakness of Mimics by helping them win those first fights and collect their abilities; whether you have iron weapons against enemy copper weapons, have a few fireball-mages along, or use siege weapons and ordinary soldiers, your mimics are the most fun when they live to see a second battle. Upgrading elite units to mimics is also a good way to increase their odds
8: Mimics need special promotion patterns: Odds are that your mimics will, at some point, run into an enemy who has only Combat promotions. In fact, most of the enemy army is likely consisting of this. This means that giving the Mimic these combat promotions is often wasting a promotion (apart from giving him better survival odds, of course). A good tactic is to give Mimics Drill promotions; early on it may be outshined by Combat, but once you have a Drill IV Combat III mimic at 17 XP, it'll pay off. Ultimately, this also means that your mimics will see more combat (as drill will minimize the damage they take when they've grown powerful), which means they'll grow stronger, which means they will see more combat, etc...
9: Nothing is prohibited: You're insane, remember? If you want to found Order and turn good; be welcome. Do you want to have an Elven Cage in every city you own? Give it a try! Want to create a legion of captured bears? Feel free! One of the things I love most about the Balseraph is that doing odds things or following crazy whims feels more "allowed".
10: Have fun! ... Point 9 sorts of leads to this. One of the selling points for the Balseraph, in my opinion, is the opportunities of fun. Of all the civilizations in FFH II, the Balseraph are those that provide me with the most opportunities for funny games.