Instructions
Get a Viera. Red mages in "Final Fantasy Tactics Advance" for the GBA are part of a rare class that's only available to sufficiently upgraded Vieras. Doublecast is a move that is only available to red mages, and subsequently Vieras. You can find a Viera to join your team deep in the town of Muscadet, in the woods. By unlocking Missionat the tavern, which involves venturing to Muscadet to search for a boy under the orders of Queen Remedi, you can access the town.
Upgrade your Viera to a red mage. A Viera must master a single fencing skill to have the Red Mage class available to her. One of the easiest fencing skills to master is Swarmstrike. This handy little skill requires a mere 100 AP to master and is learned from the Stinger weapon. The Lunge Combo is another easy skill also acquired with 100 AP. Lunge Combo is learned from the Mythril Rapier.
Acquire the Madu. The Madu is a rare weapon with 33 attack and 2 speed. You acquire it in-game though stealing it from the red mage in MissionG, "Nubswood Base." You must have a clan rank of 7, which is achieved through defeating enemies on missions and earning clan points over time. Alternatively, you can get one if your negotiation skill is up to 40. You'll need to achieve 999 AP to unlock the Madu's Doublecast ability.
Set your spells. Doublecast works with black, white, summon and spirit magic spells. Make sure that both spells are selected as A abilities so as to be sent out in battle.
Cast! You get two turns in one with Doublecast. Click your first spell -- whether it be black magic, white magic or another form -- cast it, review the damage/recovery and select the next spell. After your turn is complete, you can do Doublecast again, until your player can no longer physically perform it.
Seek weaknesses. Although Doublecast is a fancy move in and of itself, it can easily fool you into thinking you can cast anything you want. The important part is to remember your enemy has weaknesses, so play to those. Strong black magic works well on agile characters such as the Viera, while the Bangaa are easily tricked with summon spells.