Finally... The dps race is super hard and it comes down to luck with pet damage taken in the early phase (pre Tenebron landing), how many adds spawn and so on. Marksman dps is too low, so I had to go with my PvE survival spec to beat it.
For the fight itself: I preplaced an Explosive Trap and waited for the cooldown on Black Arrow to be ready before pulling. This is to get the extra slight damage from the trap, as well as higher chance to get the first Lock n Load proc quickly. Instead of doing ES > ES > AS > ES on LnL proc, I did another Misdirection instead of the Arcane Shot. I feign about the time Tenebron lands, and hopefully she gets close enough to my pet before the time I stand back up and continue. I don't want to waste globals on Misdirection and Arcane Shot on Tenebron to get it on my pet if I can avoid it. I also had a macro:

/target Tenebron
/petattack
/targetlasttarget

in order to have a button I could press without having to click stuff and lose dps that way. Aspect of the Fox should be avoided when possible, because the damage loss is tremendous and the dps race is actually that hard. I have had wipes all the way from 32k to over 300k hp remaining on Sartharion when he became immune to damage.
I prepot and precast Cobra Shot, then Rapid Fire > ES > serpent sting > BA > 2x cobra > AS > ES, and from there on it depends on LnL and haste procs etc. DPS should be absolutely maximized using the priority for survival dps. After a minute I pop Synapse Springs again and my 2nd potion. Since you have 85 seconds (I believe), the pot should be used immediate it's ready to get the full effect. When Sartharion gets to around 25-30%, I use all cooldowns on the pet and a last Mend Pet, getting to max range while preparing to kite. When Sartharion is at 25%, the other drakes go completely bananas, and you may NOT get hit by them. Once pet dies, I use Nitro Boosts while shooting like a crazy person, Disengage is used as well, and I just hope to kill him before immunity or before I die to damage from adds, breaths etc. As you will see, I die at pretty much exact the same time as Sartharion by a hit from Tenebron, and if my memory serves me well, it was for over 80k.