Whenever another creature enters, its controller may draw a card if its power is greater than each other creature's power. {G}, {T}: Add X mana in any combination of colors, where X is the greatest power among creatures you control.

10,000 Needles — Whenever this creature attacks, it gets +9999/+0 until end of turn.

You don't lose unspent green mana as steps and phases end. Omnath gets +1/+1 for each unspent green mana you have.