As an Infiltrator (an oddly, the EXACTLY same strategy is working for me in my current Nightmare PT as a Bowman) what I did was using my companions as bait. [...] In DAI I'm running with two tanks (Blackwall and Cassandra) and Dorian as CC, they keep the mobs away from me while I snipe them from the distance, works like a charm.
I used to have a sniper rifle and a sawed-off shotgun (e.g. Claymore) as an Infiltrator, as well as Adhesive Grenades. In DAI I used the Griffon Bow with Hook & Tackle, Spike Trap, Leaping Shot & Throwing Blades, in that order.
I like using extreme ranged and extreme close-quarter weapons, it's fun cause it's so busy. Just staying outside the heat and fire away with a SR/bow is fun for a while and good to have as an option, but it's the mix that makes it for me.
BioWare supports that playstyle, too, evidently by the ME3 Drell Infilitrator & DA Hunter multiplayer characters - which are obviously my favourites. Long range, mines & bonus abilities for when in close quarters.
In all my years of BioWare games, I have never tried playing a full on caster, whether it's Biotics or Magic. I've just always been a rogue for all my RPG career, and it ain't gonna change as long as devs come up with more fun abilities...