Mercs are great for expanding, considering your own troops are terrible. You'll also probably want to get an awake pretender for even more expansion, and to counter any rushes by other SCs/horrifying blessed critters. Eventually you'll need to rely on your own troops - I've had some success with using the grand princes / veliki knyazi (wonder if that's the correct plural form) with some +leadership/inspiration items, it really helps with the terrible morale. Depending on what you need, you'll want to consider carefully what troops you recruit - only the peshtsi spearmen have map movement 2. It's really, really slow to use the peasants or the slower peshtsi.
The real power of Bogarus is in its mages, though, and that power is incredible. It's not completely unjustified to call for people to rush Bogarus before it grows too strong (that's why you have the SC pretender). The power is in massing evocations and having the communion potential to cast pretty much anything outside of nature and water spells, considering all your mages have astral levels (save for the five-fold angels, but they can join your communion anyway if you really need them to). A quick and easy way to get powerful mages is to go for Evocation 4 for Thunder Strike and thaumaturgy for communions - presto, you can now use your astrapelagists for the rest of the game to destroy most things. Other spells include (but not limited to) magma eruption, nether darts, pillar of fire, flame storm, shimmering fields... You can pretty much do anything you dream up, as long as you have enough forts and money to pump out those mages.
Blood is amazing for Bogarus, I think, since it offers both a way to skip sites for magical income (it can get really ugly) and a way to summon all sorts of actually good troops and commanders to thug/cast with. It also makes an already powerful late game into a potentially ridiculous late game. You'll want to hunt in provinces with populations ranging from 5000-8000, since that's where the population is just high enough not to damage your chances to find blood slaves, and not too high that your income would plummet from the quickly rising unrest. You'll probably want to hunt with your cabalists, since the five-fold angels are pretty expensive, and a bit harder to leverage into a force to actually do battle with. Of course, if you've got a good lump of fivefold angels, you can shut down forts with unrest from instilling an uprising and the native unrest increase... but yeah, that's up to you to consider in whatever strategic circumstance you find yourself in.
I'm not a very good guide, sorry.
PS. You should really study the stuff on communions in the manual. If you don't, you'll probably lose a lot of mages to silliness (I know I did), to stuff like mixing slaves with paths X with masters with paths Y - if the master is casting in paths that the slave either doesn't have or isn't good enough in, there's a massive penalty of 4X fatigue, which really goes counter with what communions are supposed to do.