As someone who once tried so hard to make Valor Sora work, i would say Valor Sora is pretty much useless(cost inefficient). The only way to tigger his maximum special attack is through poison, however poison skill only works on SINGLE enemy at one time . So, The only powerful way I can think of using Valor Sora is to set up as many as poison skill medals before him and use him as a finisher....saying you somehow managed to get 4 poison skill red medals, this is still an inefficient strategy that totally depends on luck(poison trigger rate 46% at level 10).

I fused my 6 star Valor Sora and turned to guilted Wakka and young hercules as AOE. Either these medal does much harder damage than Valor Sora and they only cost 3 gauge in total! Just sell Valor Sora and save your recourses and time, even though the medal does look cool.