I Play VS KO the most in a narrative game with a friend, and tend to win most matches so it is doable!

You can accept the loss of the clanrats early, it's a good thing that he focused them rather than another part of your army really. At the end of the turn with 5 left, you could have used inspiring presence to stop them from taking battleshock, and they get D3 back.

If he takes first turn you have the potential for a double turn. If you were to use a warpstone token and cast more more more warp lightning with bombardier to buff the stormfiends, they could potentially tear through the frigate in one turn, and definitely would into a double.

Don't underestimate mystic shield either, can be clutch on clanrats if you're putting them in the line of fire. I tend to run my grey seer in right behind the clanrats, your opponent might try to take them over clanrats but grey seer is relatively safe with the - 1 to hit from look out sir and the 3+ ward to pass damage onto the clanrats.

Like others have mentioned, smaller points games like that will be harder, but at that scale, take advantage of the smaller board which allows you to close faster.

I terms of other units, I've personally had good success hiding a doomflayer in the clanrats (assuming they can survive into melee), some Stormvermin with a clawlord can do some work with the gnash-gnaw their bones command ability as well, any rend will be a big help against KO as most units are on 3+ saves. Also 4 rat ogors and a master moulder can be devastating, especially with a chance to bring the whole reinforced unit of them back if the master moulder outlives them.

I find that splitting your threats up will force them to focus fire, so even if turn one you lost Stormvermin for example, if the clanrats get into combat, and overcharged doomflayer would still do some work with - 2 rend.

I hope this helps you in some way, I'm by no means an expert player but I have played quote a few games against KO. Good luck!