You can jab/ f-tilt/ ps the lasers, and at low damages, even if you get hit, depending on how far away you are, she can't get the follow up.
I don't know why you would approach with a bair, especially a landing one, because that's the only real way she can grab it.
Who cares if it's invincible for a brief moment on startup, it was in brawl. Just hit her after the invincibility ends, there's about 10 before her kick comes out, the actual kick, not the auto spike hitbox. Just wait for the down b, run to the side and jump bair, if you're not used to the timing then yeah, you'll take a few hits before you learn it. As well, she only gets one down b in the air, her side b covers almost no vertical and her up b just.. straight up.
My biggest issue with this is.. why are you spotdodging with Dk, he literally has the second worst spotdodge next to Bowser.
As for Zss killing Dk, we can kill her at 80-100 with a Punch/ Fsmash.
And Zss can kill Dk at 150-160 with an Up B or a Uair at maybe 130 if we're really high up already.