Use electric shock. It'll get it to arrestable state eventually.
Also who the fuck gets their defense to 102?~!?!?!??!?!
It was a bug...
The second boss's mechanical last hand never attacked me, so I couldn't arrest it.
I could only increase my defense... and it reached 102... then the timer ran out.
But it was just a bug. After doing it several times, I finally managed to arrest all the hands.
For the third boss, I'll try electro shock and blizzard, then.
Thank you very much