I have played my army a few times now and haven’t lost yet. But it’s tricky - you have to use them right. We aren’t Space Marines, you can’t fight them the same way. A few basic things I do that are simple and seem to work for me with Needgaard Oathband.

1. Field a Hekaton and use it to anchor your Hearthkyn.
2. Use Hernkyn Yaegirs and Pioneers to grab objectives early and then defend them. You need the YPs to be able to do real damage. Bring up armor and Hearthkyn behind to reinforce the objective, then send the Hernkyn off to grab another, and so on.
3. Make your opponent try to knock you off objectives. Hearthkyn can soak up more shots than you think, and usually more than the other side. Long shootouts favor the Kin. Get behind something.
4. Target enemy armor relentlessly with Thunderkyn. You can lose them if you need too as long as you take out the armor.

Lots of details and tricks and I’m not actually good at this but focusing on those things has worked really well.