This is the problem with being the first army to receive a battletome, we did not recieve the new models and power creeps that the Orks or the Ogres got. So we suffer more and more from power creep, but that being said, it may be time to switch to a more order generalized army instead of pure Seraphon. Celestial huricanum, jade battlemage and a Thunderquake with two bastiladons is pretty effective at dealing and taking mortal wounds. Throwing some stormcast stuff into the mix gives you a few more mortal wounds to send back their way.
Specialization leads to extinction. This is a constantly evolving game system. Think through the problem, find models that are great Order support, like battlemage, or sisters of the thorn, or even cannons and other warmachines and figure out how to integrate them into your system.