Orb Shoot (also known as "regular attack") is the tree friends' main attack in the HTFA series since the very first game. The default key for performing this attack is the Ctrl key. Starting from HTFA 6 (except for HTFA GB), player characters will make noises while shooting.
Weak enemies like Goombas usually take only one hit from this attack to be defeated, while tougher ones like Koopa Muscler as well as bosses can take more. However, certain enemies and bosses are outright immune to this attack (boss example: Wheel Gator, who resists everything except for the water supply in his boss room).
It should be noted that certain enemies/bosses can also shoot orbs.