You encounter Executioner and Enchantress in Act 3 as part of the Asgard storyline.
One easy way to take them out quickly is if your team is all powered up. If so, clump your team altogether with the two villains and then use the right trigger-Y combo. This will probably either take out Executioner, or at least get him within a hit or two so you can kill him off before Enchantress can heal him. Then just concentrate everything on her.
To do this in a more conventional manner, or if you are not all powered up, just focus on the Executioner. As his health goes down, Enchantress will start to heal him. When she does this, change your focus to Enchantress. Rinse and repeat until Executioner is defeated. Then concentrate on Enchantress until she is defeated.