I'm going up there myself next week. Checkout the local website
http://Wild Trout Fishing In Assynt
This covers South Assynt, but there are also about 200 lochs on the North Assynt ticket, some of which hold monster fish, though I've yet to catch one.
Traditional Scottish flies work fine but also take Damsel nymphs - there are some huge hatches - and mini-muddlers for the bob. (the Dunkeld MM is reliable in all conditions)
A copy of Bruce Sandison's Trout Lochs of Scotland is very helpful. You'll be close to Lexy's loch - full of good fish but can be dour. Hire the boat if poss.
Loch Crocach is great fun but again you'll need the boat and a kind westerly wind - don't attempt it in a northerly. Here's a pic to whet your appetite.
Study the map, you'll find many small unnamed lochans; If there's a burn running in or out there will be fish. These waters are often sheltered when it's too rough for the big lochs.
Have fun and let us know how you get on.
PS Be prepared for midges, they're murder in August.