As someone who has been playing Carno’s in tourneys the last year, there are ways to mitigate against cannons.
-Charmed Shield on the Old one. This way if the Carno gets one shoted, the Old blood will likely survive and deny the points to the opponent. Though realistically it will take two cannon shots to take down a Carno.
- EoTG, 5+ ward from shooting is handy.
-Skrox Screen. While skinks do not stop cannon balls, Krox do. Making the opponent bounce a cannonball through a Krox (especially one that has the 5+ EoTG Ward save up) goes a long way to keeping the big boy alive.
-Target Saturation. Frankly, your opponent is more likely to shoot the EoTG.
-Use terrain. Obstacles, buildings, and hills are all your friend. The Carno is highly mobile and can make great use of terrain while projecting a threat range.
-lots of warmachine hunters. You must bring cham skinks, terradons, single sallies, etc. in great numbers to bring immense pressure on those warmachines. Players will often worry about keeping those warmachines alive and make sub-optimal decisions.