you need a physical wall, I would suggest you Poliwrath, Tangela or Golem, remind that with Golem you will have two water weakness and only seaking that resists it, you should be aware of that, with Poliwrath you will have two psychics weakness, but it seems that Lunatone and Licktung are good at it.
Since you have Agility in your Seaking, I would drop some Speed evs for some HP, or I would replace Agility for Ice Beam (it will hit hard tangelas than megahorn)
I would also think about mixed bulky Camerupt (with 252 HP and mixed evs in Atk and SpA, with toxic, flamethrower and earthquake), Nidoqueen could also play that role (if it is still NU), but always be aware if you have too much weakness.
Also, I must say, calm mind Xatu is pretty strong in the NU, if you want to replace your Lunatone.
Hope you can find a good time that isn't the standard :)