Not talking about Doomspike, it's getting HT to combo RoT that can be problematic, it depends on the size and abilities of the pulled mob. It's a thought process (see: process of elimination) more than anything else, based on the current situation.
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As Whiskey said those are two different scenarios. Preparing Impulse Drive on the "main" and apply Disembowel is a good way to start on Coil-type add like in T5 and T8. Depending on the add hp you may want to add Chaos Thrust as well. Using your "Jump" CD if up is the basic of burst strategy as well.
About Amon's slugs, it's all different because of their low hp. You can use Spineshatter and Dragonfire Dive to cover the distance "gap" and stun with the first one at the same time. Since the first two slugs are basically at the beginning of the fight I usually hold down those two jumps for them and then continue/start the TTT combo. If I remember well with my rotation one of them spawn just at the same time I have a Full Thrust ready.
The problem is that the stun effect from Leg Sweep it's a bit delayed on a "rotating" monster, I think cause of the server lag or something. So even if you use it on him the stun will take a second or less before be really effective allowing the monster to move a bit and maybe losing the HT. In a solo scenario you can correct this, on big pulls when things get messy you need better timing. Sometimes I use Leg Sweep on one mob of the group before arriving to the camp for a easy HT, but this mess up the aggro and if you got a sleepy tank can be annoying.
Back to the OP, your question is burning down a single add. The key factor to understand is how long the add is going to live for. If it is only 15 secs then it's best to HT-ID-DIS-TTT while weaving in off global cool downs. Notice there is no Chaos thrust, because the mob won't live long enough to utilize all the ticks and getting the full potency of the TTT rotation is better than HT-ID-DIS-CT-True-Vorpal.
If the mob were to live for 20secs then you'd want to do HT-PH-ID-DIS-CT-TTT. You add in the Ph and the CT cause you can get the full ticks of PH and the potency and couple ticks of CT is better than only getting an extra True -> Vorpal at the end of TTT rotation