I needed several hours until I defeated Midir. And at the end of the fight I had no Estus and exactly 0 HP (yes, that is possible, by using Tears of Denial). ... I wonder, what would have happened, if I anyway had taken on the Symbol of Avarice after defeating him and therefore died before receiving the souls. Would I have received any souls at all, just the souls I would receive without the Symbol of Avarice, all souls including the bonus for the helm or would I even have to fight Midir again? ...
Anyway, after defeating him, I had the feeling he's not that difficult if you stick to the following strategy: Just run as fast as possible to the end of his tale and hit his tale. At the end of his tale you are safe from every single attack he has until he turned or started flying for his fire attack. There are just two problems with this strategy:
1st: When he is flying or running away, it is very difficult to close the gap again. And I had big problems dodging his lazer attacks (when I was in front of him).
2nd: One hit on this tale caused a damage of 138 (so 115 hits in total to defeat him with this strategy). Of course I became greedy with the time and tried to combo hit is head (sometimes there is time for on or even two hits on his head on the way back to his tale) or his legs. This costed me all my HPs and Estus. And still, I was fighting him for almost 20 minutes when defeating him.
Anyway, if you don't get greedy and you don't as much problems as I had with dodging his laser attacks, Midir shouldn't cause too many problems. But it's not real fun, either.
Unfortunately, I have to play almost the whole game through again, to prove myself wrong :-D ... This said, I have to mention that I'm very sad, that there is no option to refight the bosses (except Halflight)!!!
PS: Shira was absolutely useless, the two times I summoned her for the fight! One time, she was just standing in the middle of the arena, out of his range while, I was fighting him and almost died. So I locked Midir to the location where Shira was chilling around, hoping that she starts doing something at all, but it ended up in her sudden death :-D