They will essentially have the same tactics and AI for the most part, but there are several variables which affect their performance. The enemy level affects how tough they are to kill and what weapons they use. As you might suspect, the higher the level, the more powerful the enemy. Generally, there are three discreet levels of weapon use.
Enemies use only the minimal melee and ranged weapons.
They also tend to use them very often. However, all enemies seem to have a limited number of these attacks. They tend to be extra aggressive, and they like to gang up on you. Gangs of S-class mummyheads are devastating. They always have an advanced weapon for their level, and they're generally too fast or immune to distance weapons.