If the copied creature had {X} in its mana cost, X is 0.

You choose which opponent or opposing planeswalker the token is attacking as you put it onto the battlefield. It doesn't have to be the same player or planeswalker Krothuss, Lord of the Deep is attacking.

Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “As [this creature] enters the battlefield” or “[This creature] enters the battlefield with” abilities of the copied creature will also work.

If the copied creature is a token, the token created by Krothuss copies the original characteristics of that token as stated by the effect that put it onto the battlefield.

The token copies exactly what was printed on the original creature and nothing else (unless that permanent is copying something else or is a token; see below). It doesn't copy whether that creature has any counters on it or Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on. Notably, it doesn't copy any effects that may have turned a noncreature permanent into a creature. If the token isn't a creature as it enters the battlefield, it won't be attacking.

Although the token is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example, like training).

If the copied creature was copying something else, the token enters the battlefield as whatever that creature was copying.