[DGDobrev]

Well, the general rule of thumb in the beginning for me is:
More HP = Good unit. Units that can take some beating before dying are much better than the low-HP units and are much more useful. Using them goes a long way towards getting the Grand Strategy medal and its upgrades.

The next thing I'm checking is:
Can they be resurrected and do I have a unit to do so?
In the beginning, units like Inquisitors and Paladins help a lot. When you get the second stone (through the quest in the catacombs of Bolo island), there is almost certain Paladin stack in Debir Magic Tower. If you have those, you may want to concentrate on a high-HP army that the paladin can resurrect and whatever is available - let's say, above 100 Health for a close combatant and above 60 health for ranged. If such units are not available... Well, I usually make due with what's left.
- a good unit that can be added to this pool are the Royal Thorns. They can summon meatshields, which is great in normal battles, but use extreme caution when using them against heroes, as most of them cast Flaming Arrow, and all plants take double damage from it. They also can't be resurrected, so protect them with Magic Shield, if you can.

Then what is left?!
In that case, I'm going for the worst case scenario tactic - Droids, because they are guaranteed on Bolo island. Paladins/Inquisitors, Archmagi, Repair Droids, Guard Droids. I split the repair droid stack in 2 so that they can fix each other (since the repair droids fixing ability is rechargable) and try to use the Guard droids stack as a main tank.

The next thing I'm checking is whether Cycploses are available on Bolo. Although they can't be resurrected, they make good tanks due to their high resistances, lots of HP and decent damage. Even if they are ranged units, you can always get them up close to the opponent. Cyclopses with Stone Skin and Magic Shield are great against melee opponents.

My final scapegoat are the Demonologists and/or Royal Griffins. Both can be available on Rusty, Bolo, and even Scarlet Wind tavern. Both can summon units that act as an additional meat shield and do not count as losses.