In The Witcher, Geralt should use the Group style of fighting against these opponents, unless they are found alone or in twos, then Fast style tends to be better. This list is pretty much identical to the list of fast opponents, but if Geralt has attained a sufficiently high level of skill, it can include almost any kind of opponent.
Feeling overwhelmed when suddenly surrounded by a large number of previously easy opponents? It is probably because you should be using the Group style of fighting. It is much more efficient (and fun to watch, Geralt can be a human blender). To switch to Group style use the C-key or click on the Group style icon. You see that things proceed much more smoothly.
Examples of monsters
Generally group style is effective against any group (fast or strong) larger than or equal to two enemies. Geralt does large amounts of extra damage (which is not displayed as floating numbers) when there are at least two enemies. Some times, this works on a group of only two enemies as well.
Listen for the steel on steel/stone (as opposed to the steel on flesh) type of noise that is made when Geralt hits his enemies, which signifies the extra hidden damage was applied.