Suggestion:
Cluster Bomb: Kills all aliens on the screen with each pulse starts with 4 pulses (kills all thirty aliens on the screen which would quickly respawn to be killed again. 120 aliens down along with a carrier if one is the field in a matter of seconds) 500 second cool down
upgrades could be:
increased shipment: lower cool down to 300 seconds , Weapons upgrade: add pluses, Thirty birds with one stone: increases resource gain from aliens killed with cluster bomb by 2 X rank level%
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The cap starts at 5 level below the previous cap. The bonus from 5 ranks+passive is enough to get over the cap. Don't forget your XP researches!