Lunk Errant "attacks alone" if it's the only creature declared as an attacker as the declare attackers step begins. The ability won't trigger if Lunk Errant and some other creatures are declared as attackers, then those other creatures leave the battlefield or are otherwise removed from combat. The ability also won't trigger if Lunk Errant is put onto the battlefield attacking, even if no other creatures are attacking at that time.

In a Two-Headed Giant game, Lunk Errant must be the only creature declared as an attacker by the entire attacking team for it to be considered "attacking alone."