Mongrels tend to be the games answer to a Challenge Run, as they are commonly seen as inferior to other light class gladiators. Having naturally lower stats in every category, Mongrels are ill-advised unless you are preforming a challenge run. If you DO chose to have Mongrels in your team, make sure to field them with an Ogre to get the INI boost.
Like Mongrel Shamans, Mongrels usually have an initiative bonus when Ogres are near. They also tend to have Backstabber and do respectable flank damage, enabling them to topple other classes that are already engaged.
Earth Affinity is a good choice for Mongrels.