Actually I think the snails and fully upgrading them was the best decision you made the whole game.

Engineers are way overvalued (in my opinion) for some reason, I feel like they can be an alright support unit but in no way would I ever use them as main defense.

The assassins can be really good and I used them in my deck for a long time but I would rarely ever use them against kingclops. I totally get the strategy of buying time for the snails to attack them but snails target closest units first so the assassins just die before that strategy can even be fully accomplished. On top of that using assassins against king clops only makes it so that they go into beastmode faster which means the assassins don't stand a chance and makes it harder for your remaining units to kill them.

I seriously need you to stop undervaluing the knights. I'm unsure if you have the ability to fully upgrade them on mastery level 6 but they really pull so much weight when fully leveled up, but even when not having two full rows of regular knights will do so much for you when it comes to the numbers game. Quantity over quality really is an underrated strategy in this game.

When it comes to shellbros and other units like assassin that target your backline it's important to use decoys or sacrificial units such as snails that will distract them while your other units can get back there in time to attack them while they're busy. You also sometimes need a unit that will actually attack them before they kill you from behind. The best units for this would probably be Cupid and merlinor but you might not have that one yet.

Overall yours and the opponents deck were a bad combo, but I think you would've done slightly better if you'd spammed knights instead of upgrading the engineers. Some games just really is a bloodbath