For there has been a child born to us, there has been a son given to us; and the princely rule will come to be upon his shoulder. And his name will be called Wonderful Counselor, Mighty God, Eternal Father, Prince of Peace.--Isaiah 9:6 New World Translation (NWT) (the poorly done Jehovah Witness 'translation')At first their claim seems legitimate, since God is Almighty, but what they are ignorant of, or ignore is that God (Jehovah as they say) is called "Mighty God" in the very next chapter of Isaiah:
A mere remnant will return, the remnant of Jacob, to the Mighty God.--Isaiah 10:21 (NWT)The Hebrew the words in Isaiah 9:6 (9:5) and Isaiah 10:21 for "Mighty God" both say אֵל גִּבּוֹר (El Gabor).
God, or YHWH, is called "mighty" (gabor) elsewhere for instance in Psalm 24:8, again from the JW preferred NWT:
Who, then, is this glorious King? Jehovah strong and mighty [ וְגִבּוֹר ], Jehovah mighty in battle. -Psalm 24:8 NWTSince 'Mighty God' is used as a reference in Isaiah 10:21 to Almighty God their claim "Mighty God means not God" is an invalid argument.