Pica is at his strongest when using haki. That is supported by the manga.
The entire reason he stopped fighting Zoro and targeted other people was because he wasn't strong enough to defeat Zoro.
How will Pica throw or stomp Zoro when Zoro can cut his golem body into pieces in an instant?
The manga showed us how easily Zoro can deal with Pica golem body. That why Pica continue to run away from Zoro during their battle.
Zoro cut the golem at the waist to separate Pica from the ground so he couldn't run away and only hide in the upper part of the golem.
Pica was never a threat to Zoro.