This one took me awhile too. what i did was i got alot of heavy troopers like say twelve.
I did not get the first guy with this strat. Then i blocked the road with the hover transport and when the second convoy came around i used the "F" and "T" keys to change the leader and prime his gernade for launch. Watch out for Beri. Then as soon as he gets close enough he will change sides and will turn over to you then put him in the TRMB and go right to the evac point. You will take alot of damage but you will make it and everything will freeze.
Hope this helps and wait till the next mission which i think is a hunderd times harder.