In MG's Ankano encounter he just kills Tolfdir and stands invincible channeling his Eye. It doesn't open at all and not react when zapped by Magnus staff.

How to fix that?

Not sure I got you.... You can't kill Ancano? You don't do anything to the Eye, you have to kill Ancano. Combo physical attacks and dragon shouts if you can use them without sweeping up innocents....

Ice form on him should do the work

Ankano is invincible.

I haven't had any issue with him being invincible on any one of 10 toons.

You use the staff of Mangus on the eye and that disables his invincibility for a short period then it comes back on and you gotta zap the eye again.

I have been using the staff on the eye gone through 5 charges complete and he's still invincible, driving me nuts. It's been like 10 minutes of him running around. Froze him did not last one second. I have got him on another character was not this difficult. Very annoying!