Dont upgrade buff beams until middle crystal dies.
Hint 1: 5 DU Buffbeam. No defense is placed on top of the bridge. Place spider and mage out of the range of ogres. Mage should still be able to heal all archers.
Hint 2: 4 DU Buffbeam. All Towers Stacked on the Buffbeam
Hint 3: slowness, lightning and strength drain aura stacked. For 3k+ aura range: Make sure the auras dont stick into the nearest spawning area when fully upgraded!
Hint 4: 6 DU Buffbeam.
If you don't care about stuck mobs replace the DeadlyStrikers with LightningTowers
For AFKing: Use a Repair Summoner (Djinn + Imp) at Hint 2