Poseidon fills up his passive tide meter by hitting basic attacks. Using abilities will decrease a portion of the tide meter. So basically, fill up your tide meter with basic attacks, then DO NOT use any abilities for 1 minute until you see this achievement pop.
Use Poseidon's second ability, Trident (
on standard controls), to make it easier to fill up your meter (it will split your basic attacks into 3 shots, and also does not consume any tide meter!).
Technically you could use some abilities and stay above 50%, but every ability except Trident will reduce your tide meter by at least 25%, up to Kraken reducing it by 50%, so I would just stay on the safe side and not use anything if you're not sure of the exact percentages.
You can see the full tide meter here, directly to the left of the health/mana bar.EASIEST METHOD:
If you play an Arena, you will fill up your tide meter simply by helping your team kill the buffs at the start of the game, then you should get your achievement 1- 1/2 minutes into the game.