My Valkyrie Tank is a piece of cake in epic mode. If you have some troubles with it, just keep spamming Air Strike with Matt (especially with Air Support), Defend is excellent (Defender for nukes, of course), Secret Weapon for massive damage, Cat Warrior and Cat Tamer are useful tools too. For Natalie, Elementarist X2, X3 are strong enough for powerful damage, and Slime Bunny auto-summon is more than sufficient.
Keep an eye for Protect and Barrier, because my Ion Cannon would be an OHKO without protection.
And, of course, when you complete a wave, boost ONLY defense and magic defense, you will need them later.
That's all for now.