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.
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.