After skull spiders spawn, they will get as close as possible to you. Once close enough, they will start to charge a mini-blast that will surround them. Do not attack them in this state, or you will be hit by that blast, and reset your combo. When a skull spider is charging, aim for another one. If there is only one remaining, wait a short time for the attack to end, then strike. Note: During later levels skull spiders will get upgraded attacks. The charge attack will be the same, however they will have spiked armor which lingers a short time afterwards.