Single trapping at a minimum is preferred. If you're not comfortable skippering from the wire in that much wind, then probably better to stay in the boat for control. Also, if the chop/swell is significant, then you may need to stay in the boat or at the very least, raise your harness height. At sissy cord is helpful for the crew.
As for tuning the boat, max downhaul is a must, as is sheeting hard when going upwind. Travel out as necessary to keep the boat in control. Also, it's real important to keep your head out of the boat watching for and anticipating gusts and waves. If it gets too much, roll up the jib, head to the beach, and grab another person. The 18 is a blast in 20 to 25+ with three on board. When it gets to be that windy, I love to just power reach with the crew on the wire all the way at the back of the boat.