LJ kill things. Kill things good. 
I like him on Offense paired with Evelyn and my +18 all-attack Melendor. They make tile damage hit hard, especially with Crit troops, and LJ firing after Evelyn is devastating, even to teams full of HOTM.
His mana debuff is also useful on Offense and has nice synergy with my standard mix that includes Proteus, and is even more useful in Quests.
In practice, I don’t find Slow mana makes a giant difference. Often by the time Evelyn is ready to fire, he isn’t far behind if he isn’t already charged.
And it works out nicely that any last tiles to charge LJ can make use of Evelyn’s debuff.