Killmonger is becoming a real force. With Usurp the Throne, he can not only dish out some serious pain, but he can also score you 2VPs that can swing a tight game. There are, however, a number of things you can do to stop him from executing his game plan.

Shoot him
Killmonger has reasonable defensive stats, but importantly no defensive superpowers. His 4 physical defense is good, but a lot of long ranged turrets use energy or mystical. Targeting his weaker stats on approach can result in him getting dazed and that (hopefully) lost activation can really hurt. The down side with this strategy is that you will be loading him up with power to explode next turn – ideally he is far away: more than his M move.
Statuses
Nobody likes having status effects on them, but they can really hurt killmonger. The big on here is stagger. Yes, stagger is probably the best effect in the game, but it especially hurts him. Denying him the extra dice on a second attack in an activation is huge for him. Watch out – in Wakandan affiliation he still has access to Wakanda Forever that can allow him an extra attack. Stun is good as it will prevent him powering up as you try to take him down from range.
Push-me Pull-me
It might initially seem that pushing him away might not be very effective against someone with charge, but making him spend those two power can often cost him two dice on his first attack as he uses his Vibranium Weapons instead of Black Ops Strike – he’ll need to start on 5 power to Charge and Black Ops Strike. Even if you don’t have a load of push effects, moving away so he isn’t in range is something that is in your control. There may be scenario reasons not to, but you can isolate him and continually make him come to you.
Got your back

For me this is the most effective strategy – have a bodyguard. Either Captain America or Okoye can redirect hits. The ability to force Killmonger to spread his attacks denies him the extra dice that really pushes his power up. Unless you are expecting a Wakanda Forever this turn, you are usually better to let the first attack go through on the intended target. Its is often the lower power Vibranium Weapons, to gain the power for a Black Ops Strike as a second attack.
Call him a mewling quim
Loki with a mind gem is 5 threat and likely to be the Usurp the Throne target. With his trickster ability to walk away from Killmonger, he can walk away from an incoming attack. If Killmonger has enough power to black ops strike twice (plus probably charging) then he can still get two hits on Loki, but that requires a huge investment of power, which means paying for Usurp the Throne is that much harder. He’d need at least 8 power – 10 if he wants to charge as well – and that’s assuming an leadership bonus from Avengers or Cabal. And with the mind stone super power, he is very likely to have to charge. Still possible, but you are really making him work for it.