It is a very simple prismtunity to achieve. Leave bihydra as the last mirage standing. I've met to condition from toppling stacked bihydras and normal unstack bihydras multiple times. The prismtunity will not occur. Either the prismtunity condition is listed wrong or it failed to activate. Please Patch. Thank you.

P.S. I am using a limited edition disc copy version 1.01