Destro and Afli are suboptimal for damaging tendons.
For max perfomance of demo following gear is very helpfull: 4 t13, moonwell chalice, either of the badge use trinkets, enough gear for 2000+ haste, everithing else reforged to hit cap and mastery.
The best priority for tendons is following:
For 1st, 3rd and 5th opening, Felguard active:
-At 10-15% of amalgamation use chalice,
-3 seconds to the end of chalice buff pot is used (best bet 3rd tendon, 5th opening)
-use doomguard on the 1st and 5th opening with 2t13, else on 5th opening (i don't use any intellect buffs for this pet on this boss, it's not worthwhile for 19 stconds of tendon)
-use meta
-use macro with soulburn, use trinket and orc buff (this should happen after death of amalgamation, second before opening tendon
-use immo aura
Tendon opens:
Inci (for 5% crit debuff) - doom - felstorm - immo - corr - hog - shadowflame - inci - inci - ....
When felstorm finishes you should change pets with active soul burn to felhunter with the active assist, it will bite tendon twice.
For 2nd, 4th and 6th opening, Felhunter active:
Chalice cd will be up shortly before tendon, so use it when amalgamation dies
-use meta
-use macro with soulburn and orc buff
-use immo aura
Tendon opens:
doom if target 40%hp+, if less don't bother - Inci (for 5% crit debuff) - immo - corr - hog - shadowflame - inci - inci - ....
In order to have meta each opening, you need at least 2000 haste, some luck and higher pririoty for fillers (inci - soulfire - hog) on amalgamation and blood.
Have log of 6,3 million damage to tendon in 25 ppl, which was the best result for demo warlock in world of logs two weeks ago
World of Logs - Real Time Raid Analysis
PS Everything i wrote works only if you main goal is to damage tendon, overall damage is horrible.