Kodo is the most badass thing you should have in your army. First off, he has 1000 life.. so he can take a beating. Second, he jacks off an aura that enhances the damage of all nearby allied and player controlled units. Great for adding a little punch to your attacks. Best of all, he can eat a target and incapacitate it.
You should always upgrade the wardrums. More damage = more easier times in battle. Unless you lose to micro or unit lacks.
As for massing kodos. I have one replay where we won using morts and kodos vs 2 teams of orc. Hilarious shit it was, I had bearads and my tinker was like level 6+. Kodos are more useful for eating t3 melee then they are the other units. Since piercing kills it, so does siege and casters would just incapacitate it.
If you want to save your units, either kill the kodo or possess/charm it. The cargo will be released and you can eat something else.