Unless it's an instanced event... then:
Lava everywhere
Fortunately the Old Ones Army is immune to most obstacles, lava they are immune to, conveyors and teleporters dont impact them, they dont trigger pressure plates, they pass through blocks and the ice wand doesnt stop then, its uncheesable.
That being said, the Old Ones Army has multiple tiers, tier 1 is Post-Eater of Worlds/Brain of Cthulhu, tier 2 is 1 mech-boss dead, tier 3 is Post-Golem.
Tier 1 contains 3 enemy types, Etherian Goblin, Etherian Javelin-er (or something) and the Dark Mage, super simple and easy to beat!
Tier 2 contains alot more, and a new boss, I dont know the exact mobs for the tier 2 but I know the boss, the Ogre. The Ogre has 3 attacks, stomp, spit and smash. Stomp does alot of damage infront of it. Spit launches a boss of snot at ya, slowing you down. Smash is just a club hit that does pretty intense damage. Ogre drops good stuffs.
Tier 3 is where it gets interesting... the final boss arrives, and she is a doozy. Bertha is a dragon, with duke fishron levels of difficulty, with tons of epic loot and attacks.
I will let you get the loot yourselves, but just know it wont be easy!
Good luck fighting the army terrarians!