Tactics for the Stronghold.

For example, 30 hippogryphs deal 3 damage to a bull from the Swamp Castle in one hit.

A level 1 Kreg Hack with 4 attack power, using 30 hippogryphs, kills 10-12 bulls of a level maximum Tazar with 99 defense. How you managed to kill three at equal levels is a mystery – shield/stone skin/curse/weakness? Does the opponent have a Potion of Life?

A friend was playing as Nila from the Tower (defense), and I was playing as Kreg Hack (attack). The levels were approximately equal, +/- 2 levels. I think I was level 21. My skills at the time of the battle were 21, 13, 3, 3. Nila's skills were 13, 11, 13, 12. I have an advantage in troops: 10 angels, .... My friend has ... 55 nagas, ...
I attack the naga first with an angel...

So, I calculated.
Angel, level 50, 20 attack, 20 defense (hero +21 attack, +13 defense)
Core Naga, level 30, 16 attack, 13 defense (hero +13 attack, +11 defense)
I took level 21 for both.
Formula:

D(sum) = [D(bas)+MD(bas)+M(of)+M(luck)] * [1-M(def)]

D(bas) = 50.
MD(bas) = (Att-Def) * 0.05 * D(bas) = ((20+21)-(13+11)) * 0.05 * 50 = 42.5.
M(of) = 0.30 * (1+0.05*Level) * D(bas) = 0.30 * (1+0.05*21) * 50 = 30.75.
M(def) = 0.15 * (1+0.05*Level) = 0.15 * (1+0.05*21) = 0.3075.

Total: D(sum) = 50 + 42.5 + 30.75 * (1-0.3075) ~ 85.

That is, 1 of your Angels should deal 85 damage to 1 Core Naga of your friend. 10 angels - multiply by 10.
I simulated the battle with your skills and levels, my Angel dealt 84 damage, I think an error of one is acceptable. What is wrong with you? 10 angels should have killed 7 nagas, I think that's okay.