I agree that the training having different conditions/bonuses are unnecessary since every mon basically boils down to going Full Atk/HP or Full Def/Spatk, seriously just focus on either the first or last
If you plan to build a mixed dmg dealer, Max Spatk 1st then use swap stone to turn Spatk to Atk then max out Spatk again
For maxing out All stats, you want to do the WarpEvo engine or Burpmon Engine to makes things easier, theres a guide on discord for that.
Now for the damage part, Happiness plays a role on your Digimons damage, If you evolve your Digimon it resets the happiness of said mon, which means you'll be dealing less damage, just tap or click on your Digimon to see it's happiness, you want the Happy Face :D with the teeth showing for "max" damage, just praise it for every thing that you do to max it out, or feed it the pink fish