Thundaga is a magic spell in Final Fantasy VII Rebirth. It is granted by either 
Lightning Materia or Lightning and Wind Materia, and is the highest-tier 
Lightning-elemental spell above 
Thunder and Thundara. Thundaga deals great lightning-elemental damage, with a high MP cost and long casting time, and costs two ATB charges. The spell causes a bolt of lightning to hit the targeted enemy.
It is also used against the player from Gorgon Mane and Hecteyes.
Use[]
Thundaga costs 21 MP to cast with a level 3 lightning or lightning and wind materia equipped. The spell has great damage output, particularly against those weak to lightning (normally mechanical enemies), but its prohibitive MP cost and ATB charge use means it should be used seldomly, ideally only to exploit elemental weakness, unless no other spellcasting option of its caliber is available. Among its peers— Firaga, Blizzaga, and Aeroga—Thundaga's reliability in hitting targets makes it strong against single enemies, with a slight drawback in its area-of-effect damage.
Thundaga outclasses both Thunder and Thundara in terms of damage output. In many situations however, players will prefer to cast Thundara, either if there are not two ATB charges available, or to avoid interruption that Thundaga's long cast time can result in. Thundaga is best used against enemies already staggered or otherwise immobile (such as after casting Stop or Sleep).
Using 
Support Materia can alleviate some of Thundaga's shortcomings and make it safer to regularly cast. Swiftcast Materia in particular can mostly eliminate its lengthy cast time, while Magic Efficiency Materia reduces its prohibitive MP cost. Otherwise, HP Absorption Materia and MP Absorption Materia are effective pairings to improve a party member's sustainability, Magic Focus Materia can increase its damage, and Magnify Materia can be a powerful way to hit multiple enemies at once with immense damage. 
ATB Boost Materia can be useful to quickly top up to two ATB charges when the player needs to use Thundaga.
In some cases, players may wish to bolster Thundaga with two support materia. This can be done by equipping two lightning materia (or one lightning and one lightning and wind materia) and pairing each up with a support materia. Swiftcast for example is effective when used alongside magnify or HP absorption, as is magic efficiency alongside MP absorption.
Using Thundaga with Aerith's Arcane Ward will cast it twice. Further using Yuffie with Doppelganger when in Arcane Ward can cast Thundaga four times, potentially overwhelming enemies.
If the party member casting Thundaga has unlocked their lightning-elemental weapon ability (e.g. Rolling Thunder for Aerith), Thundaga becomes Thundaga α, which will bestow a lightning-elemental sphere to the spellcaster for the rest of the battle (unless the player changes the element with Firaga α etc.) that will supplement small magical lightning-elemental damage when defending against certain enemy attacks (perfect-blocking not needed) and when using ATB abilities. The attacks from the "lightning sphere" do build stagger and can be critical hits.