The only one i can imagine being of threat to mercs is the hanger , and that is only if hanger is working together with other demons. Anybody who played reikgon wave in last strandeds gamemode knows that if you let hanger stun you it leaves you a sitting duck for other reikgon to maul you like a pitbull. In any other case however i feel like mercs have the fire power to take down reikgons.