Why couldn't Dante create a Philosophers Stone by herself?
The reason she couldn't is simply because of the risk. Unmarked spoilers follow.

Recall that, during their first attempt to create a Philosopher's Stone, Hohenheim was nearly killed by the process. In order to keep Hohenheim from dying, Dante used alchemy to fix his soul to another body.



Because of this risk, Dante knew that, in order to create another Philosopher's Stone, she needed a way to survive the process. That meant that she either needed someone else who would complete the process&mdash;so that she would not die&mdash;or someone who could save her after the process was complete.

Since alchemists of the aforementioned skill levels are rare, and those who possess such abilities would likely not have helped a suspicious old lady (or young lady, by the end of FMA), Dante needed (and trusted) only Hohenheim to help her complete the process.