Here is a way to handle all the enemies over 2 frames. Change "%2" to as many frames as you want to span.
global 'current'
//// This only needs to be run at the stat of the layout or when an enemy is created or destroyed.
For each Enemy
-> Enemy: set variable 'group' to loopindex %2
Always
-> set global 'current' to (global('current')+1) %2
For each Tower
- Enemy: variable 'group' equal to global('current')
-- For each Enemy
--- Distance between Enemy and Tower < range
-> Add target Enemy