The stronger your hanger gets, the more impossible that task becomes. Pilot's just aren't using javelins and rocket mortars at higher levels. Having completed the individual tasks for 23 bots, I've given up on the Aegis and Arachnos after 4 years. Changing the task has been brought up, but there are more important things on the devs plate to warrant wasting time on such a trivial thing.

Look after the little things and the big things will take care of themselves. If a task is next to impossible it should be changed as a matter of course. Broken windows.

i dont know about undercover task.. but i know that to complete task like killing enemy with autocannons you just need to put on autocannons.. by doing this you automatically decrease your hangar power and start to compete against other autocannons in sweet theory.

 In practice not, but if you at the same time do tasks for other robots, you can put on other robot thermal lances and javelin racks and longarms and whatever weakest weapon is. Your whole hangar is now sp dropped. 

If your robots already 7 stars i feel sorry for you, you are too late at completing your tasks then. If they are still 2-4 stars then you are fine. 

Use this hangar for archaic stupid tasks and another hangar for normal games, tournaments etc. Just remember to tick it so its your only hangar in play and game engine doesnt look at your other hangars to match you with enemy.

Thats the way i see and what i do about the problem. I have sp dropped hangars for tasks, but i dont put one super crazy unkillable robot to it. Sure you will struggle at 1000-2000 sometimes with autocannons lol.. but you can buy cheap 40k legendary implants for them if you use pilots of course. And you will meet tons of sp dropped hangars with legendaries at the level of 10th division. But at least you are not a newbie so you have more chances to complete your tasks there.