Multi Shot Lightning Enchanted, often abbreviated MSLE, refers to a boss monster with those two Monster modifiers. This combination of mods was extremely dangerous prior to the v1.10 patch, since the multishot caused the lightning charges to stack up their damage, causing them to deal enormous damage. Characters without high hit points and lightning resistance and/or lightning absorb were frequently killed by a single bolt.
The danger was exacerbated by the Amazon skill Slow Missile which caused so many bolts to linger on the screen that the sprite limit would be exceeded, and the bolts would turn invisible.
Other modifiers, such as Fire Enchanted or Cold Enchanted, stack their damage on top of the bolts, and increase the danger further. The most dangerous is when the monster gets the Conviction aura as well, cutting the resistance of effected characters.
MSLE Nerfed
Since v1.10 MSLE is no longer especially dangerous, since the lightning bolts don't stack up or deal any more damage than those from a normal LE boss.