As Midir is my favorite boss in DS3, I place my sign down at his fight all the time. To players summoning Sunbros for help, please heed this advice: STAY CLOSE TO THE HEAD. This fight is hard, much harder co-op than solo due to other players' (usually the host's) inexperience with it. It becomes INFINITELY more difficult when even one person starts attacking his tail or underneath him. Such attacks bait out Midir's fire, laser, and tail-sweep attacks, all of which are way harder for us summons to dodge and (if hit) survive. Add to the fact that he will be spinning nonstop just ups the frustration. The best thing you can do as a host is stick next to one of your summons (not Shira for the love of all that is holy) and follow their lead. Gank squads against Midir are fun, but a team effort is required to turn it into a victory.