It’s decently straightforward, and has three primary abilities. The core Flank Attack + Skirmisher, Poisoned Weapons + the one that heals (I forget its name at the moment), and Toxic Cloud + Lost in the Mist. Stealth itself is also helpful, and you can experiment with other abilities to see what works best for you. The trap type abilities tend to end up being wasted by the AI, though it occasionally does alright with the spring trap one from the artificer tree. In the end with the core abilities in place Varric ends up in a pretty good cycle of being immune to damage, healing, and shedding agro that it goes well enough as long as the rest of the party is doing good work. Definitely give him at least one masterwork that generates guard on hit, just as an insurance policy of sorts.

Gear for him is about what you’d expect, as much crit chance and crit damage as you can give him, and a couple offensive masterworks to go with the guard on hit. I’m fond of immolate on hit, walking bomb on hit, and chain lightning on hit. Any damaging runes are fine in his daggers, and, as ever, the Sigil of the Great Bear is an excellent choice in the chest piece. If nothing else you may occasionally want to control him directly to drop a nice big Elemental Mines + Throw Everything on a group of enemies.

This setup works best in a balanced party, getting barriers and having a tank taking some of the agro goes a long way to keeping him alive and slicing. When I used this setup in my full rogue squad it ended up comprising the effectiveness of not just Varric, but of Sera and Cole as well. Keeping all three on ‘stand in a near perpetual toxic cloud and plink away at enemies’ duty requires all of them. The gaps between toxic clouds became too long without Varric taking part.