So during training I decided to teach my Simisage some new moves. I checked his stats to see if I should go with a physical or special move set, when I noticed that his Speed stat was HALF of what it should’ve been. I reset the game and his stats were fine. I only battled a Wigglytuff and a Nochet (the one that has a music note for a head). Is there a way that I can keep his stats at a steady growth rate and not change when he gets to level 100 or do I have to manage his stats after every battle?
The short answer is that if you want to raise a Pokemon's stat, then you should feed it the corresponding vitamins and wings, and use it to defeat the corresponding Pokemon. If you don't want to raise a stat, then you should not let it defeat the corresponding Pokemon, or feed it certain stat-lowering berries. If you want to know more about stats and EVs, then you should read some Bulbapedia articles.