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You need to use as wide an aperture and as high an ISO setting as possible so you can use really low flash power to get the shortest recycle time. Also, an external battery pack will help if you set the flash to use only the pack for recycling. (You still need internal batteries to run the electronics.)
Yep, Brian hit it. The lower the power, the faster the recycle. If this is just a one time project, use triggers and borrow another flash to double them X2. Then you can drastically drop the power level on each, to allow faster recycle time. HSS, is hard on batteries, it may get you that one shot due to extending strobing length, but will not get you faster recycle time. Also, HSS, causes the strobe to drop power, something like 1/2 drop by default, compared to a full regular flash at say 1/200.
Also make sure you use a strobe that has overheat protection, some of the older speedlites don't have this, thus they affectionatelly become known as "smokers". Once the magic smoke is gone, well it's costly.