I do not attack the high ones.
I play the long game and just pick off the low end few battlegrounds where I lose few or no troops.
The level 3-10 maybe as high as 15 I attack with my free skeletons from the crypt so it costs nothing, restock and come back again if I haven't won the battle next day or the day after that..( so build up as many skeletons as you can as quickly as you can)
If I am fighting someone else with boosts and the fight is over I will attack battlegrounds while I have the extra bonus more aggressively figuring if I have a 20% boost to attack / defend I am saving that amount on every attack. I only use infantry or cavalry for attacks as I usually receive high occult or Beast in return that I stockpile for player battles. Lower are fast to generate so I never use beasts as sometimes even on a low level 8 battleground I have lost a dragon(s) against very few battleground demons so I stopped using them.
This is a good strategy for beginner players to level 40 as you may not have advanced enough to be even able to generate beasts so this way you can get them for free. But I would then not use them in battles with players as without the lost arts improvements, you will just be throwing them away against more advanced players.
If I have lots of attack /defend spells in use, like 20% unit defense bonuses and 10-20% dragon unit defense bonuses I will use all my built up cavalry/infantry and maybe even some occult to get the most out of using my bonuses until I have nothing left in those two categories.
I will fight with whatever is my highest rated defender or attacker, so if my pathfinder has the most advanced lost arts level/highest attack rating, I use and generate only that one troop to use. I also max out only one infantry ( one in defense, one in offense) so I can have a strong attack or defense as early in the game as possible rather then improving all the infantry to their max wich takes forever. Even at my current 70's level there are many cavalry and infantry and occult I have not improved at all as I only max out one of the many offered in each category.
That's how I do it anyway, hope it helps you stay strong, E.