What type of play (matched, narrative, ect) do you plan to engage in? If matched play, what amount of MPP vaule? Are you playing with any list restriction and/or taxes?
Seraphon are a top tier army in competitive matched play. We are a bit mid to bottom tier in any other environment. Seraphon have a hard time working in a toe-to-toe just murder the other army scenario. Our only viable strategies in these types of games is to spam Engines of the Gods, run a Slann, and a Saurus Astrolith Bearer and just hope we out summon and grind down the enemy before they kill off the things that let us summon.
Versus Darkiling Covens, these are my recommendations. Kill any Darklings General ASAP. His is a walking buff machine and often times the fulcrum on which their whole army is balanced. If DC unites are forced to use their own bravery then they will often times crumble. If your opponent runs Bleakswords or Executioners then hit them with Summon Starlight from your Skink Starpriest. Executioners are a higher priority target, but in general it is safest to hit the one most likely to enter in combat that turn. Try your best to tie up their Darkshards in combat. Chameleon Skinks are perfect for this, especially if your CS can be in cover while they do it.
Some generic advice is to use Seraphon's high mobility within our Allegiance ability The Lords of Space and Time to out play Darklings on the objective game. You will also want a Slann to stop enemy spells, provide buffs, and most important garner Celestial Conjuration points to summon new units or resummon units which have been slain until you expand your collection.
lastly, I reccomend looking at the battalions that Seraphon have access too. A solid beginner-friendly starting point is the Thunderquake Starhost. A more advanced, but certainly potent, Battalion is the Shadowstrike Starhost. If you are looking to get some mileage out of the Saurus Warriors then I recommend the super battalion the Fangs of Sotek.
If you provide the game type and list building restrictions you plan to use I would be happy to help you create an exact army build.
I hope this was helpful! Best of luck.
