It switched moves between turn 34 and 35, it cannot be scarf.
Also, for the Boomburst, I assume you meant on turn 30 right?
First, that's the wrong Chansey. Also, Semako's definitely was Eviolite.
0 SpA Life Orb Pixilate Diancie-Mega Boomburst vs. 252 HP / 0- SpD Base 110 SpDef Eviolite Chansey: 242-286 (34.3 - 40.6%) -- guaranteed 3HKO
252 SpA Life Orb Pixilate Diancie-Mega Boomburst vs. 252 HP / 0 SpD Eviolite Chansey: 255-302 (36.2 - 42.8%) -- guaranteed 3HKO
Both are within the range of the calc. Not sure how you ended up with a 23-28% calc though, did you make sure to alter the SpDef stat since that'd be copied straight from Diancie?