Seekers are resistant to all physical damage (Blunt, Slashing and Pierce), so every weapon type will struggle equally - hence some of the new weapons are elemental hybrids to combat them better.

My personal favourite is Himmin Afl (Lightning Atgeir), but I was already using the Black Metal one before that. The other Elemental options are a (Frost) Sword and a (Poison) Axe.
So if you want to level skills in advance, one of those is probably best.

Probably the best weapon as of now is the Fire Staff though, maybe even too much so.
Ranged, AoE, huge damage and DoT, low weight and no ammo to carry - and while you need special food to gain Eitr (Mana) which means low health and stamina, casting doesn't cost stamina, so all of it can be used to run and kite (and the Protection Staff takes care of the low health).
You'll have to start from scratch with leveling Elemental and Blood Magic though.