As derphead as I am, I can't seem to complete this challenge. If anybody would like to help out that'd be tops.
I find I usually get stuck trying to finish off the hackbox, I get lit up without any of my teammate bots helping out and rarely throwing me a revive.
It's also odd that at the end of the challenge the enemy bots are at 5k+ each in points while my team are at sub2k. Confusingz.
Gamertag is Dvs Kittenz
Cheers^
There are quite a few long and drawn out threads on the subject that might have the advice you seek to help you through it.
Popular opinion is that it's easier to complete it with a fresh character becasue the enemy bots are lower in level and don't behave as aggresively.
Once you place the hack box go behind that pillar so you're near your Command Post and do half circles around the pillar, peeking out at the general direction of the hack box on each end to keep the enemy a.i. focused on you. Just keep hacking unless the enemy engineer tries to pull it, in which case get in there and kill him (and make sure he's dead to keep him from being revived).
As for the enemy getting more XP, they're set to defend objectives and are always near them so they earn a lot more XP than yours will.