[Dedas]

The ground troops
As I wrote before I believe that the ground troops are mostly good for defense with their map move of 1. One of them even has castle defense bonus for exactly this purpose.

The Khlysts
One exception to this is the Khlyst with map move 2 and stealth, meaning that they are suited for partisan activity. The unrest causing ability also hints at this. The commander for Khlyst groups should be the Five Fold Angel for the same reason. He can also stealth preach and bless the group in battle. The blood magic he possess can be used to for buffs and offensive spells.

Cavalry
The cavalry is, as I see it, the real army for offense. Forget the slow infantry when attacking as they will only bog you down. Let them slow down your attacker instead so you can mount a successful counter charge with horses.

The summons
As most of them are flying they can be used in conjunction with cavalry to strike selected weak spots in your enemies defense. You find out which these are by infiltrating with your Five folds.

The seducers
Use them to further stall and hamper the enemies attack upon you so that your lowborn ground troops can stand a chance in battle.

The general strategy, roughly
Early game
Spend all your money on research, meaning more castles and gold producing provinces. Do not bother to buy more troops than needed to expand. Preferably: use your awake pretender to do the expanding without hampering your research and castle building budget. The choice of pretender is up to you, but why not a blue dragon or the like. At least choose someone that can expand on himself.
If someone is attacking you defend with your fast and hopefully very mobile (flying) pretender and some chaff.

Mid game
Now you should have done lots of research, mostly in the conjuration school. This enables you to have a broad base of specialized summons, with seducing and stealth preaching abilities and whatnot.
Your blood economy should also be up and running using Five Fold Angels as blood hunters (with a few Khlysts to accompany them). They should blood hunt in provinces bordering the enemy you soon believe to be attacking you, making the first provinces he conquers poor and unrestful. When the enemy really is attacking, do NOT pull back your Five Fold Angels, as they will now be used as partisans and spies, liberating temporarily provinces behind the attacking enemy army.
The blood slaves they have hunted should be used to achieve this goal. Use the excellent spell "pain transfer" for example. It should enable the FFA to take and deal damage against enemy PD, and in conjunction with some blessed Khlysts it should not prove too difficult to overcome it.

The defense
Let us look at your assets at this moment. You should have:
Many researchers.
Five Fold Angels + Khlysts (cheap in upkeep and overall)
A lot of castles.
Not much else.

Now for one of Bogaru's strengths.
Quickly shift focus from research to war summoning and military.
Use your former researchers to:
*Summon specialized elite fliers to terrorize the attacking army.
* Summon a chock army of fliers. Like the three headed wyverns.
* Forge items to kit your commanders out.

Use the castles nearest your front to:
* Build ground troops.
* Build some nice PD to be used in combination with your ground troops when defending. Do not waste much money on it when no ground troops are available to strengthen it.

The other castles should build heavy cavalry and offensive spellcasters and "buffers".

You should have all the money needed as you haven't built or paid for any military (upkeep and such) until now. But if more gold is needed fast you have your alchemists to transmute for it.

The attack.
Led by your flying pretender, the flying shock army, with a core of elites, should strike the enemy rear. A rear area in disarray to the enemy, much thanks to the FFA and Khlysts. The area is also well scouted and can be defended quickly by the same. Let your fast cavalry do the mopping up of the hostile attacking armies, thinned by seductions, starvation, cold and valiant sacrifice by your ground troops.
Obviously you should no longer build ground troops in your outer castles. Instead switch to cavalry and offensive spellcasters to strengthen your oncoming forces.

The enemy, having fought a long and costly war should now be weakened. Not only in military organization but also in research. The momentum is in your favor. Now it is time for you to relieve the ruler of this heavy burden...

The conclusion
I believe that the trick with Bogarus is to quickly and accurately switch from heavy research to heavy defense to heavy offense. This is something Bogarus is more than capable of. The hard part is to know when, as timing is critical to success. Also, knowing often means spying, so that is critical as well.