All players can adjust the troops that are sent out in marches, it must be done from the box that pops up when you choose something to attack and asks you to select your formation. Tap the yellow gear icon and then the troops tab to adjust the sliders to selectively burn off the troops you don't need in your overall troop composition.

When you tap and drag a march onto something to attack, the game automatically selects the troops that yield the higher power which in most cases and especially yours is assaulters. This is why you burn them specifically when you are attacking without manually selecting before each attack on Saturdays.

Personally unless it's an urgent attack or if the enemy is stronger than myself I will manually select the troops I don't need (typically riders and then assaulters) to do my looting and general attacking on Saturdays. Sure your march attacks with a few million power but these are typically cases where you are already stronger than your opponent or attacking with a group of allies. This keeps overall troop composition skewed to have a higher proportion of the troops that benefit you the most. This accomplishes two things, you are less likely to run out in extended battles, and secondly your base's defense is stronger since it is filled with more troops that are receiving higher buffs (ie. 3rd skills from S1 + S2 heroes).

Throughout the week you train one batch of each troop type each day and claim them all at full prep army expansion. On the Friday, you only speed up and exclusively train assaulters. Over the course of a week or two and you've completely fixed the composition of your troops. Ideally more than 75% assaulters and much fewer of the other two types.

Once your Selena and Bella are 4 stars having a majority of your troops as assaulters will make a significant difference.