Made a guild run through Heroic Mechanar last night and defeated all the bosses, including Sepethrea, which is a first for me. And we did it with one healer (restoration shaman) and four melee (protection warrior, arms warrior (me) and two rogues), which is really surprising given how much AoE is in that instance. So I thought I would follow up my initial Quick Tips for Heroic Mechanar with some further thoughts about the three most annoying mobs in the instance; Mechanar Tinkerer, Nethermancer Sepethrea and Pathaleon the Calculator.
Mechanar Tinkerer - Not only is arcane resistance useful when fighting these guys, but it also helps if you can have them going after multiple targets. Having 3 or 4 of them focus fire someone generally just causes trouble, even with lots of resistance. However, we seemed to have more success when we broke them up. Each rogue would stunlock one while beating them down and I would take on the others with my Enchanted Admantine Plate set. They go down pretty quickly. But with all melee, we couldn't take the concentrated AoE damage from standing together.
Nethermancer Sepethrea - The bane of many an adventuring group, especially in heroic mode. The real trick to defeating her is to not panic. She doesn't have a lot of hit points and she also doesn't do a ton of heavy damage like most bosses do. The real thing to worry about are her elementals. Everyone needs to be aware when the elementals are coming for them and get moving. If you let them get too close, they'll daze you, you get caught in the fire and then things go south from there. Draw them away from the main tank as far as you can. Then, when they stop to do their AoE attack, run back and get in as much damage as you can. If you notice one or more coming at you, start moving again. If you cant tell whether they're coming after you or not, move anyway and see if they follow. It's very important to NOT lure the elementals back to where Sepethrea is fighting. Hide your damage meters because that is really unimportant in this fight and will just distract you. Keep yourself alive, don't draw the elementals near other party members and do damage when you can. Also, its a good idea to spread out in a semi-circle in front of her before engaging so you can tell immediately which players the elementals are focusing on.
Pathaleon the Calculator - Similar to Sepethrea in that he doesnt do a lot of damage himself and is relatively fragile. The problems stem from his arcane elementals he summons and from his mind control ability. If he mind-controls a DPS player, you will probably want to try and fear, blind or otherwise temporarily incapacitate them so that they don't kill your tank or healer. It actually seemed to work better for us when he controlled our healer, since the healer wasn't going to kill anyone, and, as long as noone was near death, we could all survive for the 5 or so seconds it takes before the mind control wears off, leaving us free to dish out the damage. By the time we finished tearing up his first wave of elementals he was already at ~30% health and fell not long after.