One of the best ways I found is to hire in pairs. Don't just hire one junior (or entry level) at a time. Hire two or more at once so they can help each other learn your stack. At the university I work at, this is how we handle it. We hire a lot of students to help develop our application. These students are far from entry level when we hire them. Most have taken maybe one CS class at the university level.

It takes a lot of time to mentor these students, so one of the ways we handle that is have some of the more "seasoned" student developers help train. This works out very well for us, in that we have two seasoned student developers that help train the four fresh students we have. The questions that the newly hired students have can generally be answered by another student, because they probably had the same problem. While these seasoned students help out a lot, I still have to spend time mentoring and training. Be prepared to spend more time than what you think, but the rewards pay off in the end.

Also it helps if you find what the developer is passionate about and maybe find work that aligns well with that. We had a former student that once we found out they wanted to do iOS development, we put them on that task and they really shined. It does take some work to find out what they want to get into, but it has worked well for us.

As for getting up to the "senior developer" skill level, that is not something that we have baked into our student program. Once a student graduates, they can no longer work with us. It takes more than the typical 4-5 years to graduate university to hit that level.