With Johnson I got lucky having the bunkers available. Use those and back up with as many kodiacs as possible. Also have the marines enter the bunkers so they are able to fight. Have wolverines protect the kodiacs from aerial assaults and have mantis, and a Colossus's aid the bunkers. Fortify the routes and you are set for awhile. I made it to wave 27 solo. (Update) made it to wave 33 recently using the same tactics, but sending out a small 2 unit of a mantis and warthog to the edge by C, keeping another warthog or Cyclops mobile at the back center, and had 3 Sgt. Johnson's 2 at B, and 1 at A. Had the same amount of kodiacs. 2 for A and B when I could. Works pretty well. And since I have the mantis and warthog by C, the kodiac targets the land vehicles and infantry. (I could have done better by putting wolverines that way so they could target the aerial units, but I made do with what I had.