well i ve tried your method but it didnt work for me

i was short on ammo for shotgun, so it didnt worked out, but i got the two armored enemies down with the sas and received the 300 combo.
so ur save is very much useful
thank you a lot
edit
for anyone who has problems gettin the door trophy - AFTER you made the 300combo trophy - max up you guns and restart savepoint!
(double my thanks)