Hire powerful heroes and upgrade your arma, equip, gem etc. to get stronger.
After you pass the Elite instance "Fearless Man", you will get him.
Good luck.
I am at L83 just manage to get xiahou dun. I am using taishi ci, huang zhong wei yan & zhurong all at L79-80.
Try to reshuffle ur general. I let huang zhong to be 1st & taishi ci last.
Try for a few times. If huang zhong can block twice of their 1st & 2nd general. Chances of winning the battle will be much higher.
I tried around 10 times