This just doesn’t happen in the end-game. Here’s what happens when you get there.
To help, also, let’s define a comparison ratio: cheese 
being kills per mana point spent.
As someone else said - none of my troops has less than 40-odd Armour, so Bone Dragon puts a minimum of 18-20 extra skulls on the board, and will one-shot whatever I have in front, and typically chain to kill the second troop too. Having left my third troop with no Armour. That’s 2.5 dead troops for only 14 mana. That gives a cheese ratio of 0.179
By contrast, the other super-kill spells / over-powered spells:
- EK: 25 true damage to two troops, so two casts is two kills, total 28 mana: Cheese ratio is 0.071
- Maw: eat one troop and let’s charitably say create enough mana to half kill another: 1.5 kills for 24 mana, ratio is 0.063
- Mab: four casts will usually clear the board: 4 kills for 60 mana, ratio is 0.067
- Gard: three casts will clear the board: 4 kills for 72 mana, ratio is 0.056 (he gets a stilton-ish 0.083 if you manage it in two casts)
- Famine: timed right, is one kill for 20 mana, ratio is 0.05
- War (yeah, havin’ a laugh), two casts will kill anything, thats 1 for 48 mana, or only 0.021
TLDR:
- BD is the most efficient killer in end-game by a very long way
- understanding cheese is the key to success