Spoiler! About the Final Boss

Don’t be a Steve. Always use Spoiler tags.

The whole game we have been told that Tyreen want to leech the vault monsters to gain more power, so her siren power are drain life force from living and dead beings. This part is pretty clear so far. Now to the not so clear part.

When the fight between Lilith and the twins happens, she also use some sort of a blast that throws Lilith away and proceed to charge energy ball of some sorts. My first question is: What the hell is that and what it have to do with leeching power?
The most probable answer is nothing and it just there so she can beat Lilith. Pure plot convenience, otherwise Lilith would phasewalk her into the ground before she could say “follow”…

Fast forward to the final boss, as said in the begginig, Tyreen aim to leech the Destroyer and be all powerfull. But when it comes to it, there is no leeching what so ever. First of all, Destroyer would need exactly one tentacle to make a bloody puddle out of her. Second, what does leeching power have to do with her merging with the Destroyer? It just doesn’t make sense to me… Or did I miss something?

It would be a better boss fight if we would need to defeat Tereen as herself first than Destroyer… What are your thoughts on this whole situation?