The magical attacks from the sentries, empaths and Sea Sprites can be devastating. Does anyone have Soul Shield and Magic Screen ? Heal All and Haste are also very useful.If you can't handle the guards at the tunnel entrance you should head east, pick up RFS-81 at the mine tunnels, level up while you are there then head back and try again.
My favorite tactic is to enter Bayjin from the Nessie-side. That gives me a short run to the single-sentry chest and the Nice Thing that is sometimes inside it (if you open the chest enough times).