There are three major requirements for range-tanking Keleseth.

These are, in order, survivability, ease of movement, and threat. Preferably, the ranged tank will have a decent amount of all three.

I'm not going to specify the best specs for all classes that are capable of this, but I will give a few tips and tricks.

For survivability, damage reduction, self shields, and an incredible amount of stamina are all quite nice. I, personally, tend to run about 36k health specced, geared, and fully buffed, and there are definitely times I'd love some more. Damage reduction tends to come from talents and effects, so I strongly suggest taking any such, provided they don't nerf threat too badly.

Easier movement, simply put, means easier ball-gobbling. You can and will be running all over the bloody room; best get that Tuskarr's Vitality chant. Instant and/or short casts are also quite nice.

Tied up both in survivability and movement are the ball-gobbling mechanics themselves. Whoever hits them most recently has their threat, and the 35% shadow damage reduction debuff (stacking) that comes with it. The million threat is only to counteract healing threat (and even this is iffy sometimes). As such, a long-lasting, quick-ticking, instacast DoT is quite nice for holding them, as long as it doesn't cause too much damage. I tend to use a rank-1 curse of agony.

The boss threat is another matter entirely. You will spend an absolutely RIDICULOUS amount of time attacking anything but the boss. You need 3 stacks of the debuff to survive the Empowered Shadow Lances, but it's a lot more comfortable with 5 or 6.

As such, any nifty skills (like Corruption and Unstable Affliction/Immolate and Curse of Agony) that keep dealing damage while your attention is elsewhere are quite handy.

Another thing that comes in quite handy is an execute (like Drain Soul). The boss spends roughly 2/3 of the fight WELL under any skill's execute range--at 1%, in fact--which allows you to get a very large threat lead if managed properly.

Another thing to remember is that you're not well-suited to this role if your damage talents cause a massive threat reduction (e.g. Shadowform). You simply won't be able to get ahead of Caska on threat, let alone stay that way.

As an aside, melee, PLEASE watch your threat. None of the ranged tanking-capable classes have an actual taunt.