Practice to dex the last hit of his Special 1 first in your roster. Use a champion with a high block proficiency. Have a dozen revives stocked just in case to not use units. Read the description of his skills and phases thoroughly. Watch videos on YT how to beat him.
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Also use Dr. Doom if you have him.
Chee'ilth has a Grit, so that can help with unstoppable
Also, his mediums are unblockable, but you won't see "Unblockable" text appear, so be mindful of that.