The Joker/Batman Ninja

Good With

 * Gunners: Deadshot, Deathstroke, Harley Quinn, Lobo, The Joker
 * Deadshot/Arkham Origins: When teamed up with Lord Joker, the bullets fired from Deadshot’s Specials technically deal 40% Area Damage to tagged-out opponents.
 * Deathstroke/Arkham Origins: Upon achieving 200% on "Point Blank", Deathstroke's unblockable crit damage could provide effective area damage to the opposing team; that's not including the potential area effect of his infamous "Identity Crisis".
 * Harley Quinn: due to their preexisting synergy with The Joker. Harley Quinn/Animated is also extremely useful on Joker, furthering his offensive output and also being very useful on a dual Batman Ninja team.
 * Batman/Batman Ninja: His passive can help his Batman Ninja teammates making Lord Joker even more powerful.

Good Against

 * Any character or gear that can Life Drain, such as Wonder Woman/Dawn of Justice, Martian Manhunter/Blackest Night, Gauntlets of Azrael, Overpowered 5-U-93-R , Soultaker Sword, LexCorp Chest Armor V2, Black Lantern Ring, etc.
 * The Flash/Reverse Flash: His gunners damaging any of Reverse Flash's teammates followed by his tagging in would result in one of his three buffs being removed, effectively conserving specials.
 * Superman/Injustice 2: area damage does not trigger his shield, therefore granting essentially free damage.
 * Raven/Prime: The gunners can reduce Raven's health even when she's tagged out, making her more susceptible to a one-shot nuke.

Countered By

 * Batman/Batman Ninja: Being immune to lifedrain nullifies the first portion of Joker's passive.
 * Killer Croc/Arkham: His armor cannot be life drained, effectively stalling Joker's passive.

Abilities
Here are Joker's abilities. Lord Joker's SP2, "Slice and Dice", can only knockout the opponent ON THE LAST HIT (unless the opponent was inflicted by DOT beforehand), similar to Green Arrow/Rebirth, The Flash/Earth 2, and Batman/Gaslight.

Trivia

 * Lord Joker/Batman Ninja shares the exact same damage and health stats as Batman/Flashpoint and thus ties with Batman in having the highest base damage stat in the game.
 * Lord Joker/Batman ties with Aquaman/Injustice 2, Batman/Flashpoint, Batman/Gaslight, Batman/Batman Ninja, Superman/Blackest Night, Superman/Injustice 2, and Superman/New 52 in having the highest overall base stats.