Additionally, your mag is the least flexible of all ways you have to adjust the stats on a character. As mentioned you can always switch to units with higher defense or affix your units for higher defense with both lower meseta cost and lower time investment.

Do not forget that you can purchase additional mags with AC, so if that is an option, you can have a full defensive mag that you switch to when you feel like it. For players who do not have or want to get additional mags, then going full ATK (or full DEX for Braver and Phantom builds) is the recommended and most effective solution.