I have tried it numerous times hitting the targets immediately but it takes 4/5 hits always. how do you break the targets in one hit?

You can't one shot them anymore, they probably changed it a few months ago during an update.

I found a replacement but it's still slower. You can throw Ronin's knife right in front of where the second target from the back line will be. Shoot the first 3 then wait for the next 3 to appear and shoot the knife to destroy them at the same time. (you can watch my run to see it in action)

Yes, unfortunately they changed that so for 100% it might be hard to beat top places, in Any% I have a few theories that can work to skip that part.