Fierce
Violent, Intense
な adjective
If you're fierce and violent then you are fierce and violent.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
彼は猛烈なスピードで仕事を終わらせました。
He finished the work at an intense speed.
彼女は猛烈に勉強して、試験に合格しました。
She studied intensely and passed the exam.
昨日のライブ、猛烈に楽しかった！
Yesterday's concert was ferociously fun!
このプロジェクトのリーダーを買って出たことを、今猛烈に後悔しています。
I seriously regret that I volunteered to be the leader of this project now.