MortarNo question about it. The mortar is hands down, the worst card in the game right now. It does a measly 45 damage per second, has little health, and takes forever to deploy. Ever since it was nerfed several months ago. It has become almost unusable.
Hidden TeslaThe reason why the hidden tesla is so bad is because of its value for 4 elixer. It's rarely worth the extra 1 extra elixer to use the tesla when you can just use the cannon or tombstone that work just as well if not better.
Clone SpellDon't get me wrong, the clone spell is one cool card, but a zap is all it takes to destroy the weak little clones. It's just too easy to counter and doesn't bring much value to a deck. Especially when you can only use it once or twice in a match.
Dark PrinceThere is no reason for you to use the dark prince over the regular prince. Even if he gets to the tower, he isn't going to do much to bring it down. He just isn't worth it.
X-BowThis card is just isn't worth it for 6 elixer. It's much to easy to counter. Pretty much anything that can counter buildings can take it out. Oh, and if they have lightning, rocket, or fireball... game over.
Bomb TowerThe bomb tower may be good at lower arenas, but after arena 6 or 7 you will find the inferno tower much more worth it than the bomb tower as it does a pretty bad job against the meta right now, Not to mention the fact that any type of minion is pretty deadly to the bomb tower.
Barbarian HutUnless you use a spawner deck, there is almost no use for this card in any deck.
SparkySparky is one of the easiest cards to counter in the entire game. The only use for this card is in some lower arena decks
Giant SkeletonThe Giant Skelly is a gamble. If he doesn't get to the tower, then pretty much your whole push is destroyed. However, he is good on defense in lower arenas.
PoisonEver since they nerfed the poison to where it doesn't have slowness, this card has been pretty much obsolete. You're much better off using a fireball. EDIT: This card is being buffed soon to where it would no longer be on this list.