CHA (and/or IMP)* is pretty amazing in Always War. You're gonna get GGs and promotions raining from the sky, making it easier to create super-hard (top) defenders. I found it great fun to see hordes of AI attackers die, but of course at some point you need to take the fight to them, or else you will get overwhelmed. Longbows work well until Knights, but by then they can power through your dudes due to first strike immunity (and high base strength).
I don't have particular advise because I'm not very good at AW, haven't tried it all that often. But it CAN be fun. Just make sure your capital is good so it can sort of carry you economically. You'll have high unit costs, zero trade, and must do all teching yourself (unless you steal some...). Great Wall -> Great Spy can therefore be good, but on a high difficulty it's not so easy to land that one while REXing.
*On that note, maybe something like Cyrus of China would be fun? Think you get access to Cho-ko-nus then. They can cause collateral damage, and wipe melee.