Crater Elemental

• WWizards of the Coast2015-02-25
  Official Ruling
  Some formidable abilities are activated abilities that require creatures you control to have total power 8 or greater. Once you activate these abilities, it doesn't matter what happens to the total power of creatures you control.
• WWizards of the Coast2015-02-25
  Official Ruling
  Effects that modify Crater Elemental's power, such as the effects of Giant Growth or Hall of Triumph, will apply to it no matter when they started to take effect. The same is true for counters that change its power (such as +1/+1 counters) and effects that switch its power and toughness.
• WWizards of the Coast2015-02-25
  Official Ruling
  Crater Elemental's last ability overrides all previous effects that set its power to a specific value. Other effects that set its power to a specific value that start to apply after the ability resolves will override this effect.
• WWizards of the Coast2015-02-25
  Official Ruling
  Other formidable abilities are triggered abilities with an "intervening 'if'" clause. Such abilities check the total power of creatures you control twice: once at the appropriate time to see if the ability will trigger, and again as the ability tries to resolve. If, at that time, the total power of creatures you control is no longer 8 or greater, the ability will have no effect.
• WWizards of the Coast2015-02-25
  Official Ruling
  If you control a creature with power less than 0, use its actual power when calculating the total power of creatures you control. For example, if you control three creatures with powers 4, 5, and -2, the total power of creatures you control is 7.

Rulings via Scryfall