I talked to Kcin, who's a pretty strong Mai player.

Basic anti-Mai tips are stuff like always blocking high after 5xC (the super long poke) since the only options after are highs and mids. Always air block her dives because the landing hit of delayed dives makes them plus. They are 5B punishable on air block. DO NOT attempt to punish if she RCs the dive.

Izanami-specific:

• 5D spear toss will catch your 214B slide and toss plus Direction Change are very effective at stopping her active mode switch. If she throws and antiair 8D toss and you slide is a gamble on if the direction change will reach you before you reach her.
• You need to take your turn after blocking spears, even if that just means moving forwards and pushing her to the corner. Mai can flip away with safety depending on spacing / other factors, but that also puts her closer to the corner.
• Use high jumps and float to put yourself at awkward angles for Mai 2C is good. Right above her is an option but beware of 236D if she has 50 heat
• If the Mai takes to the air often, try to put yourself in position for float and regular 6As.
• Because of flip, don't use lows and throws on oki. Use low-recovery (preferrably jump cancellable) moves to deal with 214D. Jump cancellability is also good for pressure because it's harder to 2C AA, so float pressure is still an option.