So thats what we are saying, what he needs more is debuff.
Gabriel will bring Def debuff and more ATK / DEF power.
Belphegor's debuff is better than Mordred's but she might never come from the gacha.
He doesnt have any dots, blind or dizziness (assuming he will replace Rusalka)
Mordred makes more sense, and Cassiopeia will do the job during lightning event. No reason to go for Andromeda first.
Thats true. For raid event, wiser choice would be Andromeda, but she doesnt bring any damage bonus for mvp. I wouldnt go for Arthur because of Shiva's buff (and other Arthur during raid anyway). So best choice for mvp would be Siegfried. But even then, if another Siegfried is faster than you and break the stun, you're screwed.
There is no perfect choice.