You’ve got that backward. When the enemy is lower hp, you get no boost at all. So the tiles will tend to fall super unevenly in a stack, depending on the health of the target and which hero’s troops get chosen for a particular tile.
Heroes with higher health and lower defense pair poorly with Miki. It is more likely that they will have more health that a tile target.
If your heroes have 701, 790 and 825 health and your target has 700 heath, Miki’s special gives no boost.
If you manage to get a heal off late, it is very likely that none of your tiles will get boosted, since you will probably have too much health. And since specials fire before tiles, you will be left with the devil’s choice of heal and do less damage, or don’t heal and hope you get a kill with tiles.
He is simply not designed to be effective in PvP.