We should do a mock-up battle. Lets say we both have maxed out Crono and Serge, ultimate weapons and armor. All Techs for Crono and any element combination for Serge. Then we should pick an enviorment neutral to both characters, like a large field. We all should agree that if Crono hits Serge with Luminaire, Serge would be nice and crispy. However, Serge could have healing elements equipped on him, which he could use to patch himself up.
With these being said, the outcome would depend on how long it would take. Serge would obviously have the upper hand because he has healing techs, but only if the fight gets dragged out. Crono would have to take him out real quick, or it should turn into an endurance match.
But since it is nearly impossible to dodge magic, if Crono gets the first few shots of Luminaire, he wins. If Serge keeps himself alive after 4 shots (due to 99 MP cap, Luminaire costs 20, 20*4=80, one short of another shot
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..), Serge would be able to beat Crono with his own limbs while Crono justs hits Attack over and over again.
So, it depends on match length. However, I would place my money on Crono, because he probably will land those crucial shots.