Verses with the word mightily in the Old Testament (7 verses):

Deuteronomy 6:3
Hear therefore, O Israel, and observe to do it; that it may be well with thee, and that ye may increase mightily, as the LORD God of thy fathers hath promised thee, in the land that floweth with milk and honey.

Judges 4:3
And the children of Israel cried unto the LORD: for he had nine hundred chariots of iron; and twenty years he mightily oppressed the children of Israel.

Judges 14:6
And the Spirit of the LORD came mightily upon him, and he rent him as he would have rent a kid, and he had nothing in his hand: but he told not his father or his mother what he had done.

Judges 15:14
And when he came unto Lehi, the Philistines shouted against him: and the Spirit of the LORD came mightily upon him, and the cords that were upon his arms became as flax that was burnt with fire, and his bands loosed from off his hands.

Jeremiah 25:30
Therefore prophesy thou against them all these words, and say unto them, The LORD shall roar from on high, and utter his voice from his holy habitation; he shall mightily roar upon his habitation; he shall give a shout, as they that tread the grapes, against all the inhabitants of the earth.

Jonah 3:8
But let man and beast be covered with sackcloth, and cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in their hands.

Nahum 2:1
He that dasheth in pieces is come up before thy face: keep the munition, watch the way, make thy loins strong, fortify thy power mightily.