Whilst Cersei Lannister is not as powerful as she was in 1.0, she is still a formidable card. The ability to discard two cards from the opponent’s hand should not be taken lightly. It is generally the case that card advantage leads to victory; as such, removing extra cards should help bring about defeat and ruin!

Cersei Lannister combines nicely with The Queen’s Assassin who can kill a character provided he is ambushed in on an opponent who has less cards in hand. Also with Tyrion Lannister, who gleans 2 gold from her intrigue challenges. Likewise, with Casterly Rock, Lannisport, Tears of Lys and most cards that center around the intrigue challenge type. As a result, she will likely find a home in most if not all House Lannister decks.

Rules FAQ

Cersei's ability is a continuous effect that is only "turned on" when its conditions are met. If the conditions are not met, the ability turns off. That means the entire condition, "While Cersei Lannister is attacking during an (INT) challenge" must be true in order for the "raise the claim value on your revealed plot card by 1" effect to be active.

So if Cersei attacks but then leaves play (Like Warm Rain) or is removed from the challenge (Highgarden) before claim is resolved, then her continuous effect is no longer turned on. Claim is applied at step C in the DUCK acronym -- after the winner & loser are determined, but before keywords.