in the first two missions use jake, so if you keep doing those missions and saving them in different save slots 
(not the one youre proper campaign efforts are saved on) then every time he will get 300 points, and a s rank. 
do this to get levels (i got jake level 37 by doing a couple hundred times)

also if during missions with pipes in you destroy pipe seams, you get extra points with them. 
the same comes from blowing up cannon and minicannons.

if you use war room to find out which COs get the best tag team percentage 
(i.e. sasha and colin both get 130percent @trust fund@ which is max tag) 
and then pick them on deployment maps in campaign mode to get the best of tag team battles.