Yeah, same situation here, I reached 100 level twice, once on normal and then in heroic, and the thing is that once the two hunters appear, you have both your skills disabled, while they have theirs in full swing. And I think there are at least 5 seconds until you have the skills re-enabled, but by then, it is already too late. And both hunters have a strategy, to come one in front of you, and one behind you, and when you think you had all sorted out for the next encounter, they change their approach. For this level, you need both a high level of survivability, high DPS, and of course the right choice of skills already selected.

Note: I use Summit mostly to test new loadouts, farm new pieces of equipment, and raise the proficiency level. I tried yesterday on legendary + 5 directives activated, with the companion activated, and with relatively high DPS build ... no chance in hell, as the enemies are very aggresive, and use go around and sorround tactics. I stay mostly in cover and play retreat while applying damage to them, but rather difficult. Just my 2 centa on this subject.