Here are my runes and masteries whenever I jungle skarner. Most times, it doesn't matter all that much because you won't be able to proc it again, but at the same time if you damage them early then go for the big monster there's a chance you can kill all 3 at once with your AoE and not have to worry about finishing them off. Your first few clears I'd recommend doing it, but after you're finished your jungle item and its nearing mid game they will die too fast to even get proc'd sometimes.

For jungle enchantment, get Juggernaut. Don't be fooled, Skarner may have dueling potential, but at heart he's a CC tank, like Nautilus. That was the intention of his design, that's what junglers were generally intended to be. Juggernaut gives him some good stats to get beefy fast. Note, just because you're a CC tank doesn't mean you can't fight. Skarner has dueling POTENTIAL. Nautilus probably isn't going to kill anyone with his Kit alone unless he's insanely fed. Skarner, however, can go and fight other tanks or contest fights in his own jungle. Just don't be dumb about it, you're not going to win a fight against Jax at 6, but you DO have a shot against Vi, just make sure you're near full health and she's not fed. You build tank on skarner for one big reason: being able to stay alive as long as possible to keep bringing out that AoE dmg and getting those Proc's. Your enemies may be able to burst you, but after they take out that 1/4 of your health they have to wait 10 seconds to do it again. Meanwhile, you can constantly dish out those Q's and sheen empowered autos. Against bursty fighters you have a good chance to fight back. Sustained fighters like Yi, Jax, or Tryn will be fights you simply cannot win unless you have an ally. I highly stress that you do not fight early, but do so when you are at the mid game, when you have sheen and some tankiness at your side.

Take a Trinity if you're doing well or doing alright. If you're feeding, though, like going 0/4/1, then grab golem. If you 1/2/4 or something, Trinity is a decent choice, but build a bit more tankiness first then try to finish it. Golem is a decent choice on skarner, because it gives him his proc's and slows and some tankiness, but you already have an AoE slow on your E, and you have stuns/ a grab. If you're feeding, getting golem can help your ganks a lot but your dueling is hurt. Some people build golem because they prefer to do more CC and think buying trinity is just too much gold for a jungler- these are good points, if you prefer just going golem because its cheaper and you prefer to have more CC go ahead.