Try Serum VisionarySerum Visionary, MulldrifterMulldrifter, or Overlord of the FloodpitsOverlord of the Floodpits
This card gives a creature +0/-9,999 which, unless something very strange has happened, is going to kill pretty much anything
As far as Magic Arena goes, there isn't really a way to get the deck cheaper than eight rares and four mythics since Dorothea, Vengeful Victim, Katilda, Dawnhart Martyr, and Hallowed Haunting are the foundations of the deck, although turning a rare dual land into Evolving Wilds does help with the cost a bit there as well
Last, weve got Crimson Vow with another fantastic white board wipe, with By Invitation Only
Its a cheap and efficient way to dig into your deck and manipulate how the next few turns will unfold, and for more convoluted spellslinging strategy, playing cheap spells is the entire strategy
Adding the word Lotus to any Magic card instantly ups the value, and Radiant Lotus exclusive printing makes it a chase card for sure