Helmut Krabbe is a moderately strong enemy with a 100% chance to survive a lethal strike and has a fast attack rate. As usual, meatshields are good here, as he won't be much of a threat if you have a steady stream of them.
Helmut Krabbe isn't too much of a threat, even before obtaining the anti-Alien Treasures. To kill him, use meatshields and Valkyrie Cat. If you have Bahamut Cat, use him. Once he survives a lethal strike, pretty much anything can finish him off.
Later on, Helmut Krabbe becomes very easy. He shouldn't be a problem at all. At best, he's free money; at worst, he'll serve as protection for more threatening enemies.