I think the big problem here was that Twi didn't only have to beat Starlight, but do so in a way that kept the timeline unchanged. Starlight, meanwhile, could lose the fight while still messing things up enough to succeed in her main objective. Twilight winning under those conditions would require not only raw power (which she has plenty of) but also superior finesse and skill, and Starlight is both powerful and skilled enough to leverage her advantage to ensure that Twi simply couldn't pull it off. This isn't the Tirek fight, where Twi can just let completely loose without worrying about anything but victory – she is fighting with a severe handicap. But in a fair fight? A straight-up duel under equal constraints? Twilight would probably have it in the bag.

To add what Platus said, it's like having two wizards battling each other. The only difference is that while Wizard #1 is superior in terms of magical ability and knowledge of the spells he/she is going to use, Wizard #2 has set up the battlefield and knows what stuff he/she can use to try and outsmart Wizard #1. So both of them are equal in terms of overall power.