I have brought elite stacks with terrain combat bonuses usually the "open" combat. Make 3-6 of my own full 20-unit stacks. I place them adjacent to each other and surrounding the enemy city. Make sure your stacks each and all have multiple leaders, as many as you are willing to pay.
Goad the enemy out. Let them waste units attacking you. Defensive bonuses, plus your terrain bonuses, plus your adjavent reinforcement bonuses can be devastating in defence. Ive had enemies attack my 10000 unit stack with their 40-50 unit stack and they take huge casualties, as in they lose 4-5k units and I lose 200-500 because I'm so bonused up. Stack as many bonuses together in these situations.
I do love how hard it is to take Capitols for human player, feels realistic. Takes mega focus and attention to pull off.