some of the maps par time can be a huge pain to make. I can think of 3 maps I had to run 30 times with different builds before i made time (some I just used the rift scrolls and let them go because it was faster to let them pass then it was to kill them).

also heavies can knock lights over the barricades. had that happen a few times. Lets not even talk about the pain of those flyers some how getting past 30 archers (never get fire arrows on the archers).

Sometimes you have tight time restrictions, and you want to basically camp the spawn points and frontload everything. It's annoying, but once you know to play for it, it's not a big deal.

You can see remaining time (before fail) by activating 'Dispaly reference time' in the game options. If you finish before the end of cooldown, you win the last skull.