"Strong" level mobs of your own level are typically soloable one-on-one unless you get careless, but the Elites are ones that you either need to wait a level or two to tackle or get some help.
True as a "general" rule, but there are exceptions within every mob bracket. Some equal level Elites can be be solo'd, others cannot - and then some classes have an easier time with certain mobs than others. There are also some Strong mobs that are easy and others that will push you to last few % health or kill you easily is make the slightest mistake.
I love how this game can push you to use everything at your disposal and even allows you to "choose" how difficult you make a mob pack for youself.