You should do your best to have shadowflare up at all times. In some cases it's just not possible, like Titan EX, but You can have pretty constant up-time for it. As for swifting it it depends on the fight to me. Firstly if I'm under raging strikes I'll always swift it, and for titan swifting lets me stack up for weights, or dodge while he's using it. And that's a fight where if somebody dies you pretty much have to start over anyway.

And yes, always use Ruin after your DoTs are up. It won't do as much as your DoT ticks but it's not very far behind either. Only use Ruin 2 while on the move or killing a gaol/bit/gaoler or something