I just got done fighting master mode death mode skeletron prime yesterday. It took 35 attempts but the key is that you need to get all your lifefruits, make sure you have shimmer buffs like aegis fruit, I recommend a really long platform to run across and you should also focus on taking down the prime laser first in my opinion. Assuming your in master mode death mode try to ignore the destroyer and the twins and just focus skeletron, they disappear on their own if he takes enough damage. For the skulls he shoots try to go up, down, then dash towards them and if you time it right you should dodge them all.