antlers can be collected when you beat Eikthyr only (or thru cheats)
Eikthyr can be summoned even after you first defeated him, so its techniacally and endless supply of antlers

if not good euipped only spawn one Eikthyr at a time otherwise you might have massive issues

the other method is the troll farm method - find a ore deposit in the black forest and lure a troll to that place
now kite the troll over the deposit to force him AOEing and destroying the deposit wich give you the ore - i dont recommend doing this until you know what you are doing (and it feels kinda lame imho)

or maybe make a new random world - defeat eikthyr there and get the pickaxes in the required amount and bring them back into your main world to use them there (might take away a bit of the game experience but you defeated Eikthyr the legit way and earned your right to use the pickaxes)