How do I make a stat-boosting spell/attack?
To make an attack (or spell) that boosts a stat, simply select that stat as the Target Stat, and set the Cure Instead of Harm bitset, and the Allow cure to exceed maximum bitset.
A stat-targeting attack, when used in battle, will only affect the stat for the duration of the battle. After the battle ends, the stat will revert to its normal value.
For all about modifying stats outside of battle, and permanent effects, see How do I increase the maximum of a stat with an item?