Forrest's best tool standing is his leg kicks. His boxing is sub-par to be honest. If he is trying to keep it standing, he may look to avoid his kicks, due to Sonnen trying to get him down.
Forrest is very good on the ground, But Sonnen has been really training his sub defense since his losses to Maia and AS. He brought in Vinny M., as well as Robert Drysedale... who interestingly enough is Forrest BJJ coach.
Forrest also has underrated TDD, but it isnt enough to stop Sonnen's takedown. He doesnt just go for shots, we keep going for it until he gets you down. He chains his takedowns together nicely.
I see Chael getting takedowns after some good resistance from Forrest and cruising his way to a UD... Forrest will look for subs, but Sonnen will be prepared.
Forrest has also improved his control from the bottom as well, so Chael may have difficultly passing... not that he isnt content to sit in guard.
Forrest is good off his back and Chael is clearly prone to subs... But Forrest is not as good of a test for Sonnen as other LHW's would be.
Rampage was injured in his fight with Bader... In his previous fight with JJ, one of the best MMA wrestlers out there, he actually stopped some of Jones TD attempts early on. When his wrestling is on point he is hard to get down. Evans, Hendo, liddell and Lindland also heavily struggled with him in the wrestling department. Rampage's narrow boxing attack would also play into his favor, as it is not a good idea to throw kicks against Sonnen, who is trying to get you down...
I think Forrest's best strategy would be to take Sonnen down himself. Impose his size and get him against the fence. If he is on top of Chael, his chances of subbing him increase 10 fold. Sonnen's defensive wrestling does not match his offensive wrestling game... But I highly doubt Forrest would fight that way... He will most likely look to stop the TD's and beat Sonnen up standing.. He may have some success but will ultimately lose on points, based on Chael getting a few takedowns and holding him against the fence.