If the images in Welcome to the Jungle are anything to go by (they may not, since GS puts it in the house, while WTTJ is in an open field), Justin had his staff at least.
It's more likely a combination of a couple things, including that Justin underestimated Harry's strength and ability, as well as his ability to withstand pain, and Justin wasn't trying to kill Harry, but subdue him to then enthrall him.
I imagine killing with magic is easier than nonlethally subduing someone with magic.