The Horned Hellraiser

When I play Cernunnos, I'll use his Winter Aspect to support my team (Slowing low health enemies, slowing a pursuer), and I'll use Brambles and Ult the same way. This build gives you the crit ability to snag kills, as well as the lifesteal to sustain-especially combo'd with a slowing ability like Brambles.

Keep in mind, this is a support build, so hang back a bit. It's great to push and you will definitely have to in order to maximize potential, but there is something to be said for hiding behind a Guardian and tossing out Winter Glaives and Brambles whenever they are ready.

Spring and Winter Aspects are self explanatory. Spring should be used when below half health to help expedite Lifesteal. Winter Aspect is used for fleeing enemies for when you're being pushed. Summer Aspect was not covered, but can be used to melt Minions, Siege Minions, and Large Jungle Beasts.

Brambles are good for slows and AOE, use as needed. Once you build Malice and are criting often and reducing your cooldowns, you can use it more expendably.

Horn Rush is a great escape, and that's how I use it. It can also be used to ram Gods (As it says in the ability description), and chase low health enemies.

Lord of the Hunt is a great combo setter for your team, allowing them to fight unimpeded for 2 seconds. It's also great for disrupting a team fight.

Pros:
You get to be a good teammate
You can get a good amount of assists or kills (Depending)
Good Sustain
Escape Options

Cons:
Very low solo potential
This build ignores Autumn Aspect