http://www.prnwatch.com/prio_x64.exe
http://www.prnwatch.com/prio.exe
either of those will do it, i have been using priority saver for years, it also adds a a few nice features to task manager, for example; hovering your mouse over processes and services now gives detailed info.
start your game, tab out to windows find the process and select high priority, now it will save the priority you select, and whenever you use that program from now on it will always be set to whatever you selected.