The "Monster Block" type tile is what you would need to block your monster from falling.
But, yeah, I understand that for your specific case, you would like just Red Koopa are blocked and not the others monsters, right?
What you could do is have a flag "Ignore Monster Block" for your Monster's action steps, like the "ignore gravity/ignore solids/..." flags, that would make your OTHER monsters ignore the Monster Block type tiles...
We could use, for example, the currently unused "action step flashed 05".
Here's how to do that:
1) Make or modify the "MonsterBlock.asm" script, modify like that:
Code:
;; MONSTER BLOCK
;;; skip monster block if player
LDA Object_flags,x
AND #%00001000 ;; is it a monster?
BEQ skipMonsterBlock;; skip monster block if not a monster
; to ignore monster block :
LDA Object_vulnerability,x
AND #%00100000 ;; check if "Action Step Flag 05"
BNE skipMonsterBlock ;; if it IS
;; else if it is NOT
;; ETC
;; do any other checks here. Is there a bit you want to use to determine
;;; whether or not specific monsters ignore monster blocks? like flying monsters, etc?
LDA #TILE_SOLID
STA tile_solidity
skipMonsterBlock:
RTS
2) If you didn't yet, assign this script to an unused "Tile Types" in "Project Settings > Project Labels" (maybe the "Null - Walkable 11"?)
3) In "Project Settings > Project Labels", rename the Monster Action Step Flag 05 to "Ignore Monster Block":
4) Now, for your other monsters, if you want, you have the possibility to set the "Ignore Monster Block" flag (for every action steps your monsters uses):