Immortal Call / Steelskin - how they work and what gems to support them with

so, my question is: if you support a guard skill with cast on damage taken, when you take a hit, does the buff affect that hit, or just subsequent hits?

and what are the best support gems for a guard skill?

thanx.

Only subsequent hits.

I'll let someone else answer this one because I personally think CWDT is a horrible support for a guard skill. Since guard skills do not block 100% of an incoming hit, if you've taken a big enough hit to trigger a meaty one, you don't likely have enough health left to survive the next hit that hits you anyway. I personally put the guard skill on left click.

That said, there's plenty of people who do use it, so one of them can help you best.

Generally the only support you would run is Increased Duration, but it's not mandatory.