Ok, so my idea is that there could be a tower that summons a swarm of guardians to fight. The guardians can shoot air, cant attack silverfish or cave spiders, and deal 2 % additional damage per second they are locked on to a mob, and shoots up to 3 beams at once. But in return, the initial amount of damage is MUCH lower than the iron golem (10% attack of the iron golem), and their attack speed is 1 second.
For 5x5:
Level 1: 1500 coins 3 guardians Range 10
Level 2: 800 coins 5 guardians Attack buff 5% Range 12
Level 3: 1000 coins 8 guardians Attack buff 10% Range 15
For 3x3:
Level 1: 500 coins 1 guardian Range 8
Level 2: 300 coins 2 guards No Buff Range 10
Level 3: 400 coins 3 guards 5 % damage buff Range 12
Level 4: 500 coins 5 guards 5 % damage buff Range 13
http://imgur.com/a/zIOrf
For 5x5:
Level 1: 1500 coins 3 guardians Range 10
Level 2: 800 coins 5 guardians Attack buff 5% Range 12
Level 3: 1000 coins 8 guardians Attack buff 10% Range 15
For 3x3:
Level 1: 500 coins 1 guardian Range 8
Level 2: 300 coins 2 guards No Buff Range 10
Level 3: 400 coins 3 guards 5 % damage buff Range 12
Level 4: 500 coins 5 guards 5 % damage buff Range 13
http://imgur.com/a/zIOrf