Hello Devin,
Before you think of AA you should general think of your form. At the moment you have pretty much jaggies in your slime and the front looks to squarish. It looks also pretty flat
I don't really know for which perspektive you are going for but I guess it's some 3/4 perspective style. Try to give your charakter volume by better lighting.
Then you should think about your colors - try a Hue-Shift and play more with your contrasts.
Colors are just a tool, use as many as you are comofortable with, but try to use as less as you need. A organized palette is easier to use and to improve than a huge color orchestra.
Forget about Sel-Out, it looks terrible, a clear or lighted outline looks definitely better.
Improve your shading by object studies - shading tutorials won't help you. You have to observe and understand light from nature if you want to understand lighting. Once you understand it you can use it. Look at objects from different angles and try different lighting conditions, there aren't words that will help you, you just have to look - it's that simple. If it Comes to a slime look for something similar - a waterdrop for example - although if you have shading problems with massive objects too, it's a really bad idea to start with complex medias like transparent ones.
I made you a little example:
My lightsource is coming from the right front side.
The object is transparent, this means the ground will shine through the object (dark round circle at the bottom), there is also a highlight (point nearest to the lightsource), there the object is brighter, like massive medias the object itself has also a shadow (dark area right). And there is also reflected light (ligher area between dark boarder and right dark shadow area) here the object gets lighter.
~5minutes if you know what you have to do