When the Plague Horror casts Haste on a character, it is doing the player a favor. The only reason why 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 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.
One especially useful trick in the Advance version to use is for to have one of the party members kill another, and then revive them again. After one of the party members has been killed and revived, Doom will no longer affect them. Plague will then use Doom again to afflict all party members; this in turn resets the countdown and buys some time. Another useful strategy is to use Stop on two or three characters to delay their death, and after Stop has lost effect, revive fallen party members.
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, giving itself the Doom status. The Plague Horror will be instantly defeated once the Doom timer is up.
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