I had no trouble with this fight as a firebug Merc, but I'm presuming you're a healing Merc? my damage may have helped me out a LOT here. I popped my shield for the first one and let Mako heal me through it, as long as you don't stand next to Mako during the fight, she'll take very little damage during it. To start the fight, I let Mako grab initial aggro, and DfA the two. Just to get big threat, then Concussion Missiled the main target, and focused down Xan. I waited for Mako to heal us both back to full and my heat to cool down. Then I went Fusion Missile, (Incendiary Missile), Rail Shot, Unload, Power Shot, Rapid Shots, Power Shot, and repeated the last two using Rail Shot whenever it proc'd (again, my spec helped for damage output.) For the first lightning, I popped my shield and continued DPS after I moved away from him and the lightning location. (It's an AoE, just move away) Second time he popped lighting, I hit him with the Electro Dart. He didn't last much longer, but if you need another Dart to stop him again, just pop On the Hunt to instant cool the Dart and hit him again. Just remember to keep moving if you can, and keep any kind of DoTs you've got on him as best you can. He's got no heal, so just nuke him steadily, and he'll drop. Don't forget Kolto Overload and your own self heals. They help.