The spells you're going to want the most are what are called "removal spells"these are the spells that permanently neutralize your opponent's creatures by either dealing damage, keeping them tapped, or just straight up destroying them
Magic: The Gathering has held several crossovers with multiple franchises thanks to Universes Beyond, but none seem to capture the flavor of the setting as easily as Lord of the Rings: Tales of Middle-earth
If you flip a second Illuna, you can add it to hand and then mutate it onto the first one for a double trigger (and no legend rule!) Its harder to see exactly what to do with Vadrok, Apex of Thunder , since one of the best spells in Jeskai to recast Deafening Clarion will also kill Vadrok unless you mutate it under something with better stats
If a card with basic landcycling is in your hand, you can pay the basic landcycling cost and discard the card
Short of a counterspell, MTG Hexproof is the surest defence against whatever foul magics your opponent wishes to send your way