Hint 2: These barriers serve no purpose but to use those last 2 DU. If you remove them and the DSTs with them, you'll have enough DU to place harpoons there for wyverns and still afford 2 MM towers or proximity traps.
Hint 3: Minions are maybe overkill for this level, but why not! Place each group at a different choke point.
Hint 4: Place a set of 3 archers with each DST to help the wyvern defense.
Hint 5: These two won't see any action, they're just soaking those last SU; you could put them to use, but you don't need to.