There are two issues here:

 1. The ranged characters can shoot past any number of other creatures - they just have +2 or +5 to their AC depending on if you rule they have half or three-quarters cover.

 2. Your players are stupid. There are any number of ways to deal with this situation; standing still and hacking away is the most stupid of them.

A whole mess of better options are available including:

 1. Having someone behind the lead fighter cast *Guidance* on them every turn.
 2. Create space behind the lead character and have them grapple one of their opponents and drag it into that space to be turned into mincemeat from both sides.
 3. Retreating back to the previous room so the party controls the choke point.
 4. Shoving the creature in front so they can force their way into the room.
 5. Rotating the lead character so they can benefit from healing etc. and get rotated back.
 6. Using spells that physically move the creatures out of the way.
 7. Moving forward into the vacant space when you kill one.
 8. *Spider Climb* or *Wildshaping* into something with a climb speed and going over everyone's head.
 9. Physically climbing over the top.
 10. *Teleport*, *Dimension Door*, etc.
 11. And so on and so forth.