Cain is a nightmare to most fighters. His style is predictable, but how can you counter it?
Well you can
knock him out on the counter, not the highest percentage but JDS did it once, came close another few times and Kongo almost did it in every round. People say "Cain has a great chin" but that is just explaining past events, not a magical shield that exempts him from the physics of being knocked out by the force of HW blows. Mark Hunt has a better chin than Cain and Werdum knocked him out. Cain mixes up his combinations and shots pretty well here and has crisp direct boxing that's not all that easy to counter.
You can
impose your will on the feet, by blitzing forward and backing Cain up. I imagine this figures prominently in Werdum's gameplan as that's the Rafael Cordeiro signature style to begin with. Werdum's best shot in this fight is to hurt Cain in one of these blitz's and then pour it on or jump on top of him for the stoppage.
You can
keep it standing by circling and sprawling. Unfortunately you need to be a better boxer than the other guy is at cutting off the cage and the opposite is the case here. Cain also throws those devastating leg kicks (ask Big Nog) that Gonzaga exposed Werdum with and will limit his mobility.
You can win the unavoidable clinch fight by
punishing Cain before he gets those inevitable TDs. Cain has been open to damage here -- JDS landed a bunch of nice elbows and threatened that ninja choke and Kongo rocked him while his back was on the cage -- although he did wipe the floor with both of those guys. Werdum's MT is several steps up from them though: he doesn't fear being taken down and invites the clinch, viewing it as an opportunity to deal damage while fighters like JDS are just looking to get out of dodge. Cain is a much better wrestler but Werdum's all-world BJJ and high level Judo play will give him windows to finish and/or win rounds.
You can impose your will and
put Cain on his back. Cain's go-to setup is the 1-2 and then ducking in for a single and if you duck the right he's often wide open for the TD. Kids here seem to think that because Cain is the better wrestler he is somehow immune to being taken down but this is non-serious nuthuggery: it doesn't matter who you are, when you are wide open to being taken down you can be taken down easily. Werdum has shown he can take down Cormier who at HW was a better wrestler than Cain is.
Cain may stand up easily if taken down but well you scored and took some of his gas and maybe had a chance to land some big MT shots on him when he back to his feet. And if he doesn't get up... well then you won the round or perhaps even the fight. More importantly if you're on top of Cain the chance of him taking you down is much lower then if you're standing up. BJ Penn notably exploited this to win 2 rounds and get a draw against Jon Fitch (a much better wrestler known for grinding and gas tank ala Cain). Again it's a naive view of MMA that just because a guy is better at you at something in MMA you should eliminate it from your game. No one says "hey that guy is a better boxer than you, don't throw any punches this fight" because it's stupid. Likewise eliminating all offensive grappling from your MMA toolbox can only make the opponent's job easier.