Getting workable DCs for Epic Hard shouldn't be too difficult, and in the lower-level EEs, but it takes a lot of investment to get good DCs for higher-level Epic Elite quests. What are your Enchantment & Necromancy DCs sitting at, and in what content are you not landing them?
I have
a Wisdom & DC breakdown for my Cleric, to maybe give you an idea of things you might do to get higher DCs.
As for tactics, Destruction & Slay Living are my bread-and-butter trash killers, with the occasional Implosion & Mass Frog thrown in. Also, against Undead, Undeath to Death.
I do use Greater Command, either to leave mobs CCed for melees in the party to kill while I go elsewhere, or specifically against EE Storm Horns high-Fortitude-save mobs like Orcs & Giants, which I can't reliably kill without debuffing first. Cause Fear & Necrotic Ray are my two cheapest debuffs, have no save, and are the same school as Destruction so the Deific Diadem bonus kicks in.
Orange-named mobs: level drain them. Optionally also CCing them, with Greater Command or, if applicable, Hold Person. If it's off timer and the mob does not have Spell Resistance, Mass Frog works on them.
Red-named mobs: semi-pike while the rest of the party kills them. Throw Divine Healing on the rest of the party, use Vile Blasphemy & other clickies, and pretend I'm contributing by throwing Divine Wrath, my Past Life Magic Missiles, Avenging Light & Necrotic Ray. That's just me, though; some Clerics have more of a nuking focus than I do.