I just play him as a tanky support (so shurelyas, randuins maybe if I get there), since his damage is welcome but not exactly needed. I find him to be more of an initiator than anything. Rocket grab someone -> Your carry will melt them
and his damage output is high even without any AP items I find. Q+E+Ult = a pretty decent chunk of health off your target.
I might try Frozen Heart though, sounds great on Blitzcrank. But yeah Frozen Heart is great on basically anyone mana dependent
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