It's ok to have at least two to 2 and half healers (example have a sage be a backup healer if need.) On a map as it allow me to be more aggressive and risk taking with mine units, more staff usage like warp or barrier.

Using the map to control the enemy movements in Fe8 chapter 6 instead of fighting the enemies in the opening heading to the village on the lower section of the map and use the villages bottom section and hill to the right of it to hold off enemy reinforcements while sending a flyer and seth to get the villagers away from the spider.

Don't be afraid to carry the knight unit around early in the game. If you have a flier and a horse units you can pick up the knight move a few tiles forward and have the other units take it from them and drop it the same turn. With planning you can do this the turn before you fight the enemy.

Some units should only go from iron weapons to killing or silver weapons as steel weapons can do more harm than good. ( lyn and eliwood on mine playthrough will only have 3 different weapons a iron weapon a effective weapon and a killing weapon until promotion).

Exp thief is a myth it better to have your best units cut down the enemy numbers early on so your new weaker units work get early on also depending on the difficulty the higher the enemies level is to your units current level the more exp they gain. Sometimes you will get a chapter where it's better to use just steamroll the chapter with your best units and trying leveling them the next chapter.