"Blood Guards are notorious for their ability to convert a living victim into so much bleeding gore with practiced, razorlike attacks."— Monster summary(src)
Even in small groups Blood Knights can be incredibly dangerous and can kill even the most experienced characters only if not being cautious. Spells are nearly useless as they have immunities and resistances to almost everything. When encountered in large groups, it is advisable to escape. The exception is if one can cast high-leveled Fire Walls against them. Doing so and standing in front of the flames will cause the AI to walk right inside it, dealing constant, massive damage. However, a player will need a very efficient way of healing themself, and so The Protector staff is highly recommended for this sort of strategy. Another strategy is to find a way to funnel them into a more manageable group to prevent being surrounded.