Universal Cycles -- Park Tool HCW-11 Bottom Bracket Cup Wrench

Park Tool HCW-11 Bottom Bracket Cup Wrench - about $15.

If you're only gonna use it once, I'd take the bike to the bike shop & have them remove it. Penetrating oil is the best!

Yes, make sure you're turning it CLOCKWISE to get it out - it's reversed threaded. Sometimes, once I have the tool positioned I used a rubber mallet to give it a sharp blow to break the seal. That usually gets it going for me.