Old school beat 'em ups (be it longer ones like Streets of Rage™ or 1 on 1 like Streetfighter™ or Samurai Shodown™ ) typically had very limited AI. You might not call it AI at all, they were purely pattern based.
Those games typically had perfect enemies. That means, that an enemy would do something like approach you, hit you with a random attack or dodge one of your attacks 100% perfectly. To make it appear like a "real enemy" random delays are added, to give you a chance to dodge the attack or land a hit yourself.
To create the simplest form of this in construct2 you would move your enemy sprite towards the player at a certain speed, and when the distance between those two is smaller than a threshold you thought out, the enemy attacks, then cools down with a timer, and attacks again.
On top of that you add random stuff to your liking, like erratic movement, skip an attack, random delays etc.
It's perfectly simple to do, the real work is in creating all the graphics, designing the random patterns to feel "alive" and balancing it all to make it interesting for the player.