Arden Party Rock is a local party and swimming hangout and is pretty cool. I've only been there once and it was a very rip roaring high level of water at the time, so I didn't really get to see all the rock. I would recommend approaching the rock on the south bank of the river. There is probably some place to view this from the north bank, but, it is obviously easier and safer to get out on the rocks from the south bank. That would be along the B&O Road which parallels the river.
From WV 92, take CR 10 (Clemtown Rd.) to Moatsville. It's a shade over a mile. Turn left at the Moatsville Bridge, go across, and turn right to continue on Clemtown Rd. following Teter Creek on your right. Eventually, you will come to the Tygart River and a bridge. (About 0.6 miles). Turn left just before the bridge. This is B&O Rd. (CR 12/8), though there are no signs. This is a fairly wide dirt/gravel road with lots of pull off space that follows the Tygart all the way to Phillippi. The road is not bad for a dirt road, but I bet parking is at a premium during the hot summer swimming months. It's a little over 10 miles from the bridge to Phillippi. It's pretty obvious where the Arden rocks are (1.5 miles), so, I'll leave that for you, because the whole drive to Phillippi is gorgeous.
Access to the rocks is easy from the road. You can walk right out, however, a word of caution about swimming here. There have been many deaths here due to a heavy under current. I don't mean to be gross, but I spoke to a gentleman who told a story of a young man who was supposedly an excellent swimmer, who was swept down under and came out in three pieces down stream. I observed that there is substantial flow emanating from beneath the rocks along the shore, and if a person was sucked into something like that, there will be no tomorrow. So, be careful.