The original rules do have this covered, the hero is simply expected to voluntarily fall into the pit, take damage, and then fight the monster at a disadvantage, but I don't find that a particularly satisfactory method, hero players are forced to take this unappealing option to keep the game moving and avoid a stalemate, or they choose to remain on the square before the pit and force the EWP to make the same unappealing choice to keep the game moving. A more fun option is the following special rule:
Special Rule: Jumping Attack wrote:If you want to jump a pit but the only landing square is occupied by an opponent then you may carry out a 'jumping attack' action. Roll to jump the pit as usual, failure results in you falling into the pit as usual, but success means that you can roll to attack your opponent and if you kill them (or push them back if you are using pushback rules) then you land on the now vacant landing square, if you fail to vacate the square then you land in the pit.
This feels more fun and heroic to me!
= white skull, one "hit"
= black skull, one "hit"
= shield, cancels out one "hit"
