Elspeth, Storm Slayer isnt just a token generator
Putting a counter on a creature during your end step won't cause the ability to trigger
This can clear the board or just burn your opponent out of the game, whichevers more convenient
Thats 105 possible combinations
Various cards can add further power to the deck, such as the Goblin Anarchomancer, which makes green and red spells cheaper by one colourless mana, the Mage Slayer, which promises unblockable damage, or the Rhythm of the Wild enchantment, which protects your own creature spells from counterspells
Its nice because its a lot cheaper than Sphinx of the Guildpact, but you can never actually activate it in time because there is no point prior to the beginning of your upkeep when you get priority