I find catapults work best as they are +30 per unit. If you do not have the capacity to build them then attack with as much as you have and be prepared to lose a certain amount. Generally if you attack a crater with 2 dragons defending and you are lev 2-3 on your castle, you will lose @ 50 swordsman.

Thanks for the feedback.

Eventually as your Cities upgrade to Level Fives you lose less troops.
I have found sending Archers seems to cost me nothing at all, but there is no doubt that catapults are King.

Is there a manual or help which say the units defence or units attacks?
How Many piquets do I need to kill 150 cavalries?

The thing is, swordsman count as one unit. So if you need to attack a dragon, which is 20 units, you'd need 21 swordsman. The 20 swordsman to kill the dragon, and one to make it back. With 150 cavalry, those are 4 units each. So think of it more as 600 units rather than 150 cavalry. So about 20 catapults would do it, considering they're 30 units each. Hope I helped y'all out.

Does this mean that battle mecanics are just about numbers in this game?

That seems good calculation thumb rule...
Once I sent all my troops to capture a mountain and they end up killing all my archers around 250 and lOt of other troops.. . But few cavalries died...
Shouldn't cavalries and swordsman die before archers as they should lead and archers should be behind them.. .

So I wanna know if there's a way I can find out which troops am I more likely to loose against whom as well.. . So I don't use them.. .

Im pretty sure that u lose troops in battle in the same order they are seen on ur units page