1. BEFORE YOU ATTACK YOU MUST SEND A PIKEMAN TO SEE WHAT BALUR HAS FOR YOU.
2. In terms of amount of defense or attack of balur's units you need to find the type unit that the enemy had the least defense for, I witness that Infantry often works well for it, as Balur has four types of units, each is good at defending a certain type like cavalry, Infantry, Occult and Bestiary. I find that Infantry works best because the highest defense against infantry is 60, compared to 130 for the occult. Find the best unit type and proceed with more of that type than any other type.
3.Optional: if you want very accurate calculations, get a calculator and a notebook, write down the number of units Balur has(look at step 1) and Multiply the defense of each unit type by the number of units Balur has, do this to all the four types and then add the defenses together, so you get four rows and two columns, place the unit type in one of the columns and the Defence in the other column. With a unit type and the Total defense, then find the unit type that Balur has the lowest defense for, that's the unit type you choose, send most to all of that type of unit, and voila!!!
You got a victory report waiting for your inspection.
IMPORTANT: Read all the sections in this post to fully understand, and take advantage of what I have said.
What I realized:
I have witnessed many times where quite a big number of my griffins have died in a level x battleground, but when you attack high level battlegrounds like a level x+20, there are loses but not as much, if you compare the enemy units in each battleground with the stuff gained, you see that you gain more overall with high level grounds.
I have noticed that in higher level battlegrounds it is very beneficial for a player to have a ton of units rather than have powerful units,when you use the most basic units like the paladin and pikemen for offense and The dwarf and Archer for defence. You use a low train time, low cost, and highly efficient units. It can counter battlegrounds with high numbers of units.
When fighting defense battlegrounds it generally gives you more units than if you play offense, it helps when you are short on units and want to gain some more.
Higher level battlegrounds give you MUCH more units, so just attacking low level battlegrounds will make you lose more than you gain while the higher level ones are pretty much a gamble so you either win lots of units, or you lose a ton.
What you use High offensive units for:
The high defensive and offensive are best for defending a castle or attacking a opponent with other high damage units. The carrying capacity on those troops like wyverns tend to be high and therefore best used for castle raids(battlegrounds do not give you a significant amount of resources).
General strategies:
Try to join a league and have them become friends, after you can let them manage the defense in your castle and you can use all your own defensive units for the battlegrounds.
When you join a league, if they are big and good, then you should be able to get a 3 wyverns every 24 hours, so offense is the way to go if you want to get into higher level battlegrounds and so you can progress quickly.
Hope my advice helps 
