I don't think you'll ever really be able to make the AI stand up against a good player unless it completely cheats. Having better AI will certainly help but players will always find a way to cheese it.
I don't play much captain myself but wouldn't it be best to only have one respawn as a captain? Maybe three or so, but then the most efficient way would still be to rambo the first two deaths.
Yeah this is what I'm saying. Dead Captain should mean 1 or all of the following; morale loss, respawn timer, and limited lives. Those three things would change the entire meta of Captain mode. The Captain needs to be something players don't just willingly throw away.
There are DEFINITELY some things they could do with the AI to improve it, though. Like not letting more than 2 troops lock onto an enemy unit at a time... so the Captain can't just lead all your troops away from a fight in a conga line as he runs away and spinny chops them down 1 by 1 with a glaive.
It's also just slow... the AI troops do not move at full speed, captains of the same unit type are much faster. Their decision making is slow, between striking or block they often drop their hands completely and just sorta stand there.