Not FE related, but I made an AR code to change the default HP in stamina mode. If you’re using Dolphin, you can right click on Melee in the game list, click properties. Go to the AR codes tab and click Add New Code. Call it whatever you want and paste the code in the Code window and hit Save.
300 HP
021b86f2 0000012c
500 HP
021b86f2 000001f4
999 HP
021b86f2 0000012c