because charging points are subtracted in the later levels, for the last level:
take NO extra infantry
place the trenches at the tippy top of the circle
cavalry goes behind them
heroes beside cav
bird under cav, boosting heroes
elites as far beside the bird as possible with the extra spawn rates
then you exploit the game until you get 2 rolls right and are good with timing charges
step 1:
light the trench IMMEDIATELY. this must not fail. restart the level if it does.
step 2: wait for the flag to spawn. it must spawn within charge range of the infantry.
whatever infantry does NOT get the flag spawn must be used to shield the bird later, so charge him towards the bird (for the other guy charge the flag). if the flag spawns anywhere else (it shouldn't) restart.
step 3: since enemies cannot move through burning trenches, wait for the trenches to go out. Then for the cavalry to get hit.
step 4: charge. keep the troops tight and try to pincer the king. move the bird behind the elite infantry. if another flag spawns, and you can't kill everything now, try to collect it and then kill everything (in the same charge)
step 5: now use dragons breath on the cav (bundled together) to max out their health and kill stragglers if there are any.
step 6: cross your fingers.