What ever you decide to anchor with make sure its solid, to include drowning rigs. If not, when Mr. Loutre decides he wants to go home he will take your entire set into the bayou. They are a strong animal and know how to use every ounce of muscle they have. Last year I had a double crossed staked #2 long spring set on a coon run, the ground was soft, but not that bad. A big male otter tore up the ground and went home with my trap. I found one stake and a trail heading off in knee water deep into the swamp.
I don't know where you trap or the terrain but I trap a lot of otters and I do the following:
If I use a 220.280 or 330, I make sure the trap is staked or cable off to something solid. If you tail catch a otter he will roll and pull. I also put a swivel near the trap just for this reason.
Foot holds I try drown them. I will use a drowning rod or run a cable. Make sure its anchored solid. Also, you want the water deep enought to drown the otter.
Live catching I make sure my trap have a couple of swivels to keep them in the trap.
All of my drowning rigs are double staked or cabled off to small tree or root. My drowning rods are double staked. If the ground permits I use a earth anchor on my cables.
Its not a otter is some kind of mystical creature with unearthy strength, its they are all muscle, do not give up and will twist over and over and over....