Battle Troop and Enemy Design

I need help with making engaging battles, and keeping players invested in battle.  I've learned the basics of damage types and making enemies weak against certain attacks, but players tend to plow over everything I throw at them.  I've managed to throw curve balls with the bosses and scripted events but I can't rely on them for regular random encounters.

So far I have come up with custom states and racial strengths for a family of monsters to share that offer an advantage against certain actors.  I've also designed a skill to detect enemy weaknesses with yanfly instant cast to avoid wasting a turn to develop a strategy in a fight.

Are there any tutorials geared at making battles interesting, or how to design enemy trash encounters?  I feel like I am missing something crucial that I didn't glean from my own play experiences.  

What do you mean "trash encounters"? Random battles?

Yes random battles.

The monsters that as set to appear after a certain number of steps on a map.  

They generate experience/wealth for the player between bosses and cut scenes acting as filler.

I know what they are, I had just never heard of them referred to as "trash encounters" before.