Besides three incredibly strong active abilities the brand new Nicol Bolas planeswalker has the most overpowered passive in War of the Spark. Being able to use any other planeswalker's abilities, including opponent's, is just outright ridiculous.
This in combination with The Elderspell is a surefire winning combo. By the way, for this to work you don't even need your opponent to play planeswalkers, you could just destroy your own for those sweet eight loyalty counters on Nicol Bolas.
Of course, if there is a legendary creature on the battlefield, then you will need to find the way to remove it, too. But that is not a problem if you run Spark Harvest.