Contrary to popular belief, when the Plague Horror casts Haste on a character, it is doing the player a favor. The reason it fails so often is because the characters have a high magic evasion stat and the multiplier prevents the target from benefiting. Haste and Slow have no impact on the Doom status timer, and therefore this strategy backfires on itself by allowing the party more turns to attack. The exception to this is there is no Haste status to speak of; Haste affects the ATB bar, and the Doom timer is completely separate.
The key is to kill Plague Horror as quickly as possible. Rydia's Flare, Rosa's Holy, and physical attacks from Cecil, Edge, and Kain are the best bets. An added strategy is to cast Stop on Rosa so she does not die, and then revive the whole party with her.
Another useful strategy is to have Rosa start the battle by casting Reflect on a party member (preferably herself), so that the opening Doom spell is reflected. And because there is a character without the Doom status, it will keep casting the spell, resetting the counter indefinitely.
The Holy Lance is earned after defeating the boss.
plague is any virulent and highly infectious disease that can cause an epidemic or even a pandemic.A