This works for me... or at least, this is how i do any sets with 21s

Use an actual curling bar firstly do a set on a lightish weight for yourself, straight after you're done bang out 1 set of 8 hammer curls on the heaviest you can do, then next set a slightly heavier 21s weight and try keep the same hammer weight. If you can manage a 3rd set then congratulations it aint easy and you probably wont do alot else that workout

Your arms look pumped without gear on this great if youre going out on the pull