Toy Soldiers

There are two campaigns to this game. Campaign and campaign+
(12 levels to each side) You only need to get the general's ranking on the first 12 campaign levels. NOT ON CAMPAIGN+. I found the only way to gaurantee I earned this ranking on every level was to make sure no enemy troops made it in to my toy box. It didn't matter that some of my units were destroyed or how much money I made as long as no enemy troops entered my toy box, the rank was mine.
EDIT: like rogue commented below this would be easiest done on the casual difficulty.

The biggest tip I can give for this is to get a lot of bonus multipliers. Towards the end of the campaign, it gets difficult to buy better units because money is tight. Usually what I did at the start of the level was place a howitzer and upgrade it to level 2, then I get in (up on dpad) find and guide the shell (hold rt) to where the soldiers are gathering up during the readying up and count down phase. It’ll net you some good money to place more units.

The other tip I can give is know what are the best unit placement are. i.e. chemical near the entrance of toy box. Leveling everything to 3 isn’t a good idea. I usually don’t upgrade my machinegun and chemical past level 2. If I have multiple howitzers, I’ll have a mixture of level 2 and 3. Speed is better than more power. The most useful unit is the mortars at level 3 because it covers a lot of area. In general, all five units should be used. Some times chemical or anti-air aren’t useful.