Lodium: If trash respawn at certain Tiles then i woud just try occupy those tiles myself with some characters
this way im blocking further spawning of trash
unless the game has some way of spawning the trash annyway or the game crash due to bug/error or its not actully possible to occupy those tiles
RyaReisender: Occupying the cell works as strategy, but then your have to continue with less party members and the characters you left behind can't be leveled. And if there is a sudden "back attack" event or anything, they are alone and most likely perma-die. Sometimes the spawn points are also in attack range of the boss so he'll start attacking you if you occupy them (which you don't want if you still want to reach optional goals before defeating the boss).
You cant use all the characters against the boss annyway and the characters you can select to use on the map are also limited.
Its been a long time since i played any fire emblem game but it was impossible to max all the characters in one playtrhough
especially since some of the characters you wont recruit untill later so if youre taking all the characters you can level
youre doing it wrong. You need to prioritize only a few characters that you will level each map and then keep rotating those you want to level the most
some of the more easy characters to level will be max leveled early on and be damage sponges.
If tiles are near the boss, then just chose some characters that soak up alot of damage.
Heck you can even place a healer nearby thats not possible for the boss to reach
This way you can have one tank or two to soak up damage and healer trhow some heals while you are hunting equpment or characters to recruit.
Dividing the characters inb to two teams or more can help achive this and then you can dicede when to go for the kill on the boss.
Back attacks are not really sudden, they are predictable the way they happen the same way at a speccific time
If you have played the map once and the character you wanted to level died to a back attack then you know when and where its going to happen.
Sure, some of the characters will die alot untill you learn what the game code will do for each map but thats just the nature of Fire emblem style games.
if you cant be bothered to trial and error your way trhough the game theres is probably a guide or Walkthrough
somewhere
But wheres the fun in that?