My thought would be Horror Spam with Reinforcement Points. Let the Stalkers constantly waste damage on 1W models that split to replace losses so that he can't clear objectives, especially since they're troops. Also, add in some Flamers and Exalted Flamers either in the Warp or on the table. Ultramarines tend to want you to do something so that they can counter it. Don't play their game.
Don't charge into Overwatch. Stop and shoot the daylights out of them at point blank range. Flying Screamers over them to chop off some heads would be fun too.
Make him waste a TON of his army's abilities and reduce the value he gets for his points. If the Spears charge YOU at that range, Flamers of Tzeentch will just melt them in Overwatch.
On a similar note, bring Mutaliths, Defilers, and Vindicators. Lots of guaranteed damage at closer ranges that he has to charge in order to deal with and can then assist with counter charges or throw out the kind of Overwatch that makes him nervous. Vindis and Mutaliths with a screen of Horrors, Warpflamer Rubricae or Flamers of Tzeentch will be frustrating to a UM successor. You'll have to carefully manage HOW you close, but I think you can do it.