You have so low energy and heat stats, generally the mech have ~300 stats of energy regeneration/ cooling and ~600 in capacity .
Also you don't need a second nightfall if you have a spartan .
( i repost because it have a little mistake not correctable with edit )
I propose a stats analysis to verify this ,
The mercy was 19 weight heavier than annihilation, less resist drain and more damage but it have exactly the same minimum damage ( ~250 ) with highter max damage ( + ~135 ), so according to my calculation with the comparison of resist drain the annihilator inflict +60 damage ( without counting the difference of damage) so because of that it bring back to 250 - 497 damage for mercy that to say the mercy inflict average 38 only more damage for each hit than annihilation .
But it make no sense to compare these 2 weapons for this reason : they don't have the sames roles ;
The annihilation inflict constant damage at the 1 - 2 range, it is used because it does not push .
The mercy inflict decent damage and potentially push the opponent so that it can be hit by a medium range weapon .
So i propose to compare with a more similar weapon : The rock recoiler
The rock recoiler inflict damage just below that of mercy average, have the same weight than annihilation (-19 than the mercy ) but it have a cost.. at first sight the rock recoiler seems to be better, it's depends but generally the rock recoiler would be better .
For the better melee weapon i suggest this ( in order of the first and second best weapon ) :
Hugger weapon ( that bring you closer to the opponent ) : Rock polisher
Normal weapon ( with constant damage, no push or pulling ) : Disintegration
Pushing weapon : Rock recoiler