Communication
Any half-decent controller should have a reasonable understanding of your aircraft performance characteristics, but if you are given an unreasonable target then for goodnesss' sake say so.........otherwise some jerk in the training section will tell all the new controllers that you can make it.........and a few years later when those same controllers have left the shopfloor to work in an office you will find that the impossible climb/descent profiles have been written up as procedures in new airspace design models. Talk to the CAA [ not NATS ] via your Chief Pilot if this is a recurring problem.