Without the NP drain guy, who is nasty but only has 300k HP total, the Archer is just a normal servant. I think the best tactic is to burn down Enigma fast. Support Cu Alter was great here - they have so many fancy abilities when both are standing they don't necessarily attack that much. Once it's gone, you get a ton of NP from the field effect and Archer's multi-hit attacks. I left my Lancer Vlad in the back row so he came out at around this point and finished him off.