So instead of using your turn to attack your opponent directly, rather use it to get rid of their vigilance creatures
But even if you do not want to plan your whole deck around Behemoth K, he is just strong card in general and will be included in many Lightning-Decks for sure, especially since you can search him with [10-086C] Aldo, [9-063L] Gabranth, [11-072R] Tyro and another Card in Opus XIII: Crystal Radiance I also like the idea of running Behemoth K in an aggressive Lightning-Deck with Cards that constantly treating your opponent and trying to deal 7 points of damage very quickly
So at worst you have a 6/6 samurai, but at best you hit a living weapon, job select card, For Mirrodin!, Meteor Sword, Embercleave, and the nasty Nazgl Battle-Mace
Each card has a rule that needs to work exactly as written, without being interpreted wrong