The size of the second spell you cast determines how big of a spell you can steal, so you should look to cast spells that are free or reduce their mana cost, like Force of Will and Dig Through Time
Looking at it, this card is essentially a bulkier version of the Gyarados EX from Mythical Island, boasting 210 HP (instead of 180 HP) but having the same 4x Energy, 140 DMG move
The coolest thing that Ive seen someone do with March of Swirling Mist is exiling their Misthollow Griffin and phasing out some of my hatebears so that they could go off with Food Chain by casting the Misthollow Griffin from exile
Special Summon 1 "Gusto" Tuner from your GY
As you continue to venture deeper into the dungeon, you will often be offered a choice of which room and triggered effect you want to choose next: Until finally you reach the end of the dungeon, completing it
Explore, Scaled Nurturer, and Karakyk Guardian ramp lands and mana for early dragon deployment