It absolutely is not a premium box. Premium box has a far high occurrence of legendary items.
The best way to farm the boss isn't with heat or energy builds though. Takes too long to whittle away. What you want is just to max out your hit points to get the most value out of the repair drops, and use new physical damage like the nightfall. Two nightfalls on legendary/mythic level cut through the PVE mechs like soft butter, minimizing the time it takes you to clear the run.
At level 80 I have base 100 fuel, using 12 fuel per hard run (hard run is 95% success rate unless I only get 1 repair item), but it still takes a while to run that many times. Especially if you do it more than once per day eating up 100 fuel, and more if you time your level up to refill your fuel at the right point.
You'll have a lot of epics, and eventually a lot of money... but none of it will help you in serious PVP because the best new items only come from premium boxes. That is, the weapons that only have legendary/mythic power levels.
But at least this way I no longer need to waste precious tokens to exchange 1000 tokens into 1.5m credits anymore.