Hey!
About your first point: I don't feel like it's always mandatory to play with mercs with frail builds, there's a few ways to counter enemies that want to close in on you.
The bleeding effect makes it so that enemies take more damage if they move, so it's really good to cast on enemies that are about to dash, probably good to put it on a long range spell.
There's also the rune of forces, which pushes enemies away, the range is really small (like only 1 or 2 steps), but at higher spell quality the amount you push them is enough that they won't be able to reach you in a single dash action.
You could also try to play with traps, but I haven't experimented with those too much to give advice.
Oh, and a swiftshade with the shadowform trait is great at killing multiple enemies one by one. It's a huuuuge power trip, I really recommend you try it at some point!