These turrets are very poorly programmed; they always shoot either pick targets at random will, or automatically attack a weakened enemy at the REAR of the enemy echelon.
Level 6 - Plant a charge on each end of the platform, then plant your last charge on the support beam, planting them all as far down or out as they will go. When the two soldiers and the tank cross over your charge on the right, blow the left charge, then the right one, and finally, finish off the bridge with the last charge.
Level 5 - This level is hard to ace, considering the extreme amount of timing needed. Plant two charges on each side, one on the top of the firstmost strut (From that end of the bridge, perspectively) and then one on the base. Detonate the lower-right charge -just- as the first truck crosses, then blow the opposing one, then the last two charges on the tops of the bridge.
*Level 3* - Plant charges on the points where the arch crosses into the bridge's platform. Wait for the soldiers to cross the bridge just past the point of detonation for the first charge, then blow it, and then blow the other one quickly. ||| *Level 4* - Follow the same instructions as Level 1, but the slower vehicles and larger bridge give you a little more time.
*Level 1* - Plant 2 charges on each of the very far edges of the top halves of the bridge, then two more on the lower halves just below. Detonate the charges to the left first, then the others as the squad rushes across. ||| *Level 2* - Place two charges on the bases of the suspension wire columns and two more on the support struts. Again, blow the leftmost charges then the rightmost, doing so quickly but steadily. (Stars mean extra charges left over.)
I highly dislike the entropy of some of the levells. They are why I always die... Too often will I be on the complete East of one level, and the next level offers the only two slots on the far West side. It may just be because I have a high-rated computer that runs levels quickly, but I'm just saying.
When fighting the boss, let your troops build and allow your enemies to kill each other force. This way, when it comes time to fight the final legion, you'll have lots of troops to use.
Any reason why the "Piercing Darts" upgrade for the Dart Monkey looks suspiciously like the symbol for the perk "Stopping Power" from the Call of Duty series?