Could you give an example of such an encounter? Bearing in mind that any Goblin that comes within 15' of the cleric will die instantly. I would also stipulate that it has to be fun for the players and playable in a short time frame.
Because I agree with your stipulation that it has to be fun for the players, I have to add a preface to the encounter I'm about to use an example: A large part of what makes an encounter fun for players in my experience is that the encounter means something to their characters - there is a purpose to it other than the obvious purpose of being there so the players have game-play. That also means that the fun-factor is impacted by ephemeral qualities not easily put to text. Lastly, the example encounter is one that happened at my table and my players did enjoy, though I do not doubt my use of the Mob attack rules in the DMG was a significant factor.
Environment: A large chamber with numerous passages leading into it (the one the players entered from, and eight others at different angles around the sides), cluttered with piles of mining gear, overturned handcarts, and scattered bits of ore and rock. A single barred door at one end.
The enemies: 160 zombies; about 40 start in the chamber between the party's entrance and the door. The rest are scattered about 60 feet down the various other passages and will begin flooding in as the rounds proceed.
The goal: Get through the barred door (which needs to be smashed open; AC 15, 27 hit points, expected resistances and immunities of an iron-banded wooden door barred with a thick timber) because that NPC the party is sick of is doing something dastardly beyond it and needs to be stopped.
The enemy tactics: Surround characters, grab them, shove them prone, deal damage with hits after that.
Final note: Other things before, and other things known to be after, this encounter also shapes the fun of it - resources already used are not available, and any resources used here will not be available in the following confrontation with the nasty NPC. And to directly address one last thing; I don't have to bear in mind that coming within 15' of the cleric means instant death because A) there isn't a cleric in every party (my party doesn't have one), and B) not every cleric has that spell and the spell slot to use it that they are willing to spend in this specific encounter.