How many are you doing? If it's, say 8, then take 1 spike and dupe it. Then hold down 'shift' while you spin the dupe 180deg. Off-set both to the outer edges of your shield. Then group them together. Then dupe the group, and rotate the duped group 90deg in place. Now you have 4. Group all and rotate the duped group 45deg. Now you have 8. And so on. It's trickier with an odd amount of spikes, but still doable with some planning. Hope this helps! :>