I can do around 90 runs in Powerslave in 24 hours + skull quests. So, 4 SOT refills will take me to 750 cosmos battles.
I don't use all free SOT in a day, i think i lose 30-40 of them.
I think using ALL free SOT and not do all skull quests should be enough for 750 cosmos battles. But i recomend not to skip skull quests at all.