Getting yourself stronger is a matter of having equipment matching your stat, increasing your main stat in the market (increasing the stats of the specializations will help your defense), upgrading the level of all of your Haremettes as well as the affection level of your battle team, and using boosters. The battle system still has an RNG element to it that can't be avoided (hence why you are losing to other charm players with lesser stats) and with the introduction of Levitya, hardcore players who have her are at a distinct advantage.