TL;DR Build lots of low-tier units for the first wave or two, try to tackle each wave with 2-3 armies with a total of 1000-2000 army power to get away with minimal losses. Quality>Quantity, Warden can only deal damage outside of enemy Zones of Control so stay in melee with it. Burst their army down or heal through their burst, keep your units at least a hex away from each other as the Warden does 1 hex AoE attacks, they're weak to shock damage but the golems reflect half of non-physical damage taken. The Wardens always spawned near my primary armies and attacked within a turn or two unless I ran. Waves started in the first 20 turns and happened every 10-15. Good Luck!
This achievement took me 78 turns to get. The first warden spawn was about 10-15 turns into the game and happened every 10-15 turns afterwards. IIRC, the first spawn is 200-300 army power weaker than the ones that follow, a phase beast may have been left out of the first spawn or it was because of veterancy given to the units on spawn.
Edit: The gold wonder that you're given a mission for after meeting with the sorceress that called you to the planet does not stop the spawning of Wardens, I'll edit again once I complete the mission given afterwards.
The Warden's stack consists of: The Warden (Tier 5 Mythic Unit, very tanky ranged caster), a Phase Beast (Tier 4 Shock Unit, has displacement and can teleport/phase), 2 Bronze Golems (Tier 3 Polearm Unit, has a 1 hex cone attack that weakens & spawns with "Mirror Veil" which reflects half of non-physical damage back to attackers), 1 Carrion Bird (Tier 2 Fighter Unit, flyer), and an Astral Wisp (Tier 1 Battle Mage, can stun melee attackers when attacked).
The Warden's stack can range from 700-1100 based on how late into the game it is, my final wave had all Champion ranked units. I'd wager the best strategy to fight the stack is to go a Materium build, aim for high quality units with heavy physical damage, defense piercing, and/or shock damage (all but the carrion birds are weak to schock damage). All of the Warden's attacks damage enemies in a 1 hex radius around the target so you may want to stay away from using chaff.
In combat versus the Warden, you'll want a couple shielded units to hold the Bronze Golems & Carrion Bird in place while you focus down the Phase Beast and get some units into melee range of the Warden. The Warden can't put out any damage while in an enemy's zone of control so it is forced to take attacks of opportunity or sit idle. If you bring a decent variety of unit types you'll tend to get through combat unscathed. Don't attack into the Golems with heavy non-physical damage, try not to stun any of your melees on the wisps, and the rest is simple enough.