Man, he got at Optimus not once...but THREE separate times.
So ok the first time Optimus was weak and them boys was beating on his ass from the moment he came out that barn...so by the time Lockdown got to him he was running on fumes. We'll give a pass for that right.
2nd time he was distracted fighting Galvatron and the bumblebee clone making it easy for Lockdown to blast straight to his chest from 200 yards down range. Basically a blindside so he got an excuse there too.
That last time tho...lawd that dude was whippin Optimus Prime's ASS!! He needed humans, Autobots and the hand of God to save him from that massacre. I was sitting there like this dude's power ranking must be off the charts.

I was like so a bounty hunter got more juice than a Prime?

Being a Prime is like being a glorified coach then, you just there for leadership and rally abilities.