In-universe: Mickey was so adamant about SDG leaving the battle to him and the FF gang, so when SDG jumped past him and into the fight itself he was caught by surprise for a moment, afterwards he followed after them, but when he caught up to them they had already defeated Demyx. (That is my head canon)
Real-world: What Sephiroth0812 said. The battle really is not that difficult, unless you a playing a low level challenge, which was not the original intent for the game. The Water clones really are what most people have difficulty with, due to having to defeat 10 clones in 10 seconds, but with the timer increased to 15 seconds in FM, the fight litterally just got even easier.
I personally never found the water clones a real problem, when they are summoned I have usually dispatched them all within 5 seconds, so the only time I have ever had any real trouble with this fight was during my level 1 playthrough, took me nearly 4 hours to beat him, but I digress.
One last thing. I know how everyone feels about this fight because it would give me a really hard time as a kid, but after I got 2.5 when it came out and played it, Demyx only did the Water thing one time. Any reasons why this might happen? I was really happy then
That move is not an obligatory move during the story, if you can either defeat Demyx fast enough, or know how to loop him, he will never call forth his water clones. In your 2.5 playthrough you just managed to defeat him before he could summon a second time. Congrats
