If you are having problems with the missions here is a few things that might help:
1. Never upgrade your barracks. Ever. They suck, don't do it.
2. Upgrading a ranged tower is better than building another.
3. Cannon upgrades are your late game, fully upgraded cannons are the best towers in the game, after you stabilize with some upgraded archer and mage towers, get those cannon towers rolling.
 "Kingdom Rush Frontiers"){kind=icon}
 "Relic Of War"){kind=icon}
 "The Enchanted Cave"){kind=icon}
 "Electric Box"){kind=icon}