Cheeky chariot challenge combo

I took my Chaos army for an outing to GW Newbury last night.  I have now bought a winged Daemon Prince to lead them, for some Monstrous Flyer goodness.  It is still being painted and I haven't worked out how to fit him in my case yet, so I was using my Nurgle Sorceror on a bike as HQ.  I also ended up with Invisibility as my psychic power, so my unit of T6 Nurgle Bikers also got a very tasty cover save and could only be hit at WS 1 in combat.

I was playing Necrons and came up against a rather nasty combo, which ended up shutting me down.  He had two Overlords in Command Barges, which are apparently now chariots.  Anyway, I got in charges on both in separate assaults to avoid getting Hammer of Wrath from them.  I managed to wreck both chariots with krak grenades.  Each Overlord survived and was then fighting a unit and they issued challenges.  If I accepted the challenge, he then used Mindschackle Scarabs on the accepting model.  I failed all of these rolls, which meant the single model in the combat was hitting itself, but my other models were locked in combat and were unable to attack anyone, due to the challenge.  Refusing the challenge meant I was then struggling to wound him with basic troops, especially with his 2+ save.

I guess I need to read up on the new chariot rules to find a counter.  I suppose a cheap tar-pit unit to tie him up in combat would be one method.