In my multiple, MULTIPLE hours of waiting on the Derelict Leviathan, The amount of generic Centurion HVTs far outweighs any of the specific HVTs required for triumphs. To add onto that fact, There is one particular HVT that I have noticed has such an unbelievably low spawn rate, I have defeated its counterpart several times over. The Nightmare of Tlazat, Fed to Beasts has NEVER spawned for me, unlike Nightmare of Uaoc, Spine Breaker, who has spawned for me in the Pleasure Gardens about 6-7 times over the span of 10 hours (Possibly more or less on both metrics). As of writing this I have had 2 generic Centurion HVTs spawn and defeated. I currently feel like this isn't a possible triumph.