Remilia's ability allows her to steal a card from anyone whose attack she avoids. This has a strong chilling affect on other players, attacking becomes a risky proposition.
Her spell card, Spear the Gungnir, never misses its mark. Getting a Bomb as Remilia guarantees someone is losing a life.
If you're playing as Remilia, keep playing mind games with your opponents. Make sure they know that if they attack you they could lose big time. Your spell card can't be avoided, but you can only play it on your turn. Make sure you time it right to spoil someone's plans.
If you're playing against Remilia don't be afraid to attack her. Play your cards in the right order and even if you get stolen from you won't miss what you lose.