* In "Do A, then do B", B happens after A. These things happen in sequence, not simultaneously. A is required for B, but NOT vice-versa: if A does not happen, then stop; even if B cannot happen, you still do A. * In "Do A and B", both A and B happen simultaneously. BOTH A and B are required. If you cannot do both, you do nothing. * In "Do A, and if you do, do B", both A and B happen simultaneously. A is required for B, but NOT vice-versa: if A does not happen, then stop; even if B cannot happen, you still do A.[10] * In "Do A, also do B", both A and B happen simultaneously. Neither is required for the other to occur. * In "Do A, also, after that, do B", B happens after A. These things happen in sequence, not simultaneously. Neither is required for the other to occur.
Valkyrus says "negate the attack, then end the Battle Phase.", so it falls under the first one of these categories. Which means, as someone already mentioned, since it couldn't negate the attack due to PEP being unaffected, it also couldn't end the battle phase.
Its also the reason why you cant Maxx C when under shifter or Ariseheart. The card cant go to the graveyard so it cant be activated. Thats the more common form of this coming up. But you can still Ash because it doesnt have that part of its wording.
Thats why certain cards wording is so important. Cards like Forbidden Droplet use the wording choose (which is really rare) which allows it to break through most protections. Same with Zeus, the card uses the word send, which is super powerful.