Remember that multiple wounds are halved, so generally you need 3-4 cannon shots to kill it. And impact his are great. I ran 2 stone horns a few OFCC's back. If you have an iron blaster, they stonehorn will not draw the canon hate. They are great at grinding through infantry, and they also look cool. People often call monsters cannon bait, but sometimes they forget that their might be more important targets.
What I am getting at is that this is one of the most durrable monsters in the game, and while yes, people will shoot at it, they will generally forget about shooting at more important stuff and still not kill your stonehorn before it gets stuck in.