As far as i understood your major problem with your current battlesystem is that a player with a highleveled powerful troop in the front row couldnt be beaten by the enemies who stream in one by one because they cannot fully make use of their quantity over the players quality.

It lays at hand, that you need a way to make units who are constantly attacked will eventually die or get knocked out.
One possibility would be to remove the medics, could make the battle system a little more boring perhaps?
Or you might limit the amount of hitpoints those medics can heal. for example they can only heal a percentage of the units current hitpoints.
In example: a unit which is only hit once and not really damaged can easily be healed up to its full max hitpoints, in contrast a lethaly wounded unit with only very little hitpoints left couldnt really be healed by medics. it would need constant healing of multiple medics to bring the unit back to a decent amount of hitpoints, most probably this wouldnt succeed because of ongoing enemy attacks.

Another possibility might be that your units in the front rows lose stamina while in battle and once their stamina drops under a certain amount they need to slip back into the back rows to regenerate. now the former back rows will go on with the fighting.
To enforce this you might give penalties if units lose all their stamina, like a temporary attribute loss or even a stun.

These possibilitys could help to put the focus on a mass of average units instead of a small amount of high quality units which can solo big amounts of enemys.
There might of course be other ideas of the same kind or combinations of those which will work out as well.