Ive only done fenrir solo as bst and its not bad on normal. He is a lot like tojil in that the timing of his tp moves is easily predictable, allowing you to not get caught with your pants down mid ready move or something during a tp move. I tried to do bst, cor duo on D and got wrecked by poor timing on my part around 53%. I needed mulsums as reward timer wasnt fast enough. I have a good DT set but not perfect either so that would help some but even maxed out, id think you would need mulsums at times still. The biggest issue is using ready moves at the right times, preferrably while out of range of the dispel/1hr moves.
Im not the best bst out there, not even close, but i have some decent skirmish sets with pet acc, atk, DA, DT, capped reward pot/recast etc, so if you are a top end bst you may have an easy time with it on D, but that was my experience on D with a cor friend. Good luck if you try.