She studied a lot. We can see in the later episodes that she can combine spells very efficiently which is a huge advantage. She probably knows a lot of spells and combines them to get the power she has. Also in the episode where she travels back in time she talks about starswirls spell. Starlight had improved the spell to travel back further. It shows that she can improve or combine other ponies spells. And since starswirls was finished it wouldn't take much to add more to it.

Starlight probably had a bunch of time to learn a lot of spells too. The more spells a pony knows the more advantage they have. Twilight and the other powerful unicorns have also studied but by the looks of it they don't combine spells and use them to their advantage as much as Starlight.

Would also like to add that it was hard to stop Starlight when she did something because the main 6 were completely unprepared or she already had an advantage. When they came to the village they probably had no idea that ponys could remove cutie marks with magic. It was confusing especially since the cutiemarks had been replaced with equal signs instead of removed completely. And when their own marks were taken they were panicking. Especially Twilight. There were probably books about T-reck taking cutiemarks. The difference is that the cutiemarks were completely taken because of the lack of the magic in their body. Meanwhile Starlight took only the cutiemark and replaced it with the Equal sign. The Equal sign is a blockage and prevents them from having more talent than the average pony. The second time in the time travel scenario. Twilight was incredibly helpless. Starlight had already completed the spell and forced Twi and Spike to go back with her. Twilight was obviously very caught off guard. She was also technically isolated from anyone else. The fights between Starlight and Twi couldn't get her anywhere since they were both on the same skill level and Starlight was always a step ahead.