Twixtor, Twixtor and again Twixtor.
In addition to using Twixtor, keep in mind that for super slo-mo, Twixtor can't work miracles. You have to begin with footage shot at a high frame rate,
And failing the above suggestions, you could try Twixtor.
Jut as a double-check, have you used the true interpretive frame blending options (liike the Time Warp plugin) in AE? If you have, and are not satisfied, Twixtor is your next best option.
We see this topic often so you can search for many related threads.
There are only a few tools that do this well and they are expensive both in cash outlay and time required to test, explore, fix and render.
The single best time and money saver for time variable footage is to know what's going to be needed befofre the shoot and to make sure the scenes are acquired at appropriately high frame rates.
Good luck with Twixtor, please come back and let us know if that worked for you.