How to increase siege attack?

Developing heroes, using gold and X heroes, sending more troops (barracks, hero skill 1 and 6 and some techs add more troops).

For buildings, war rooms give a 30% might buff if you use a single troop type in an APC.

Fighter, Shooter and Vehicle tech as well as APC tech will also help you grow.

Specifically for Attack, deploy your eden specialty points to RED RIGHT (seige attack).