There are several reasons that make Trishula Vyuha the best strategy to combat against Kalakeyas. First of all, the army of Kalakeyas is comprised of 100,000 soldiers while the Mahishmati has only 25,000 soldiers in their army. In a direct combat, it is a clear defeat as Kalakeyas are strong, ruthless and outnumbered Mahishmati soldiers by Four to One. It would be a foolish decision if they plan to kill the entire army of Kalakeyas. They need to avoid direct faceoff with Kalakeya army and end the war without killing their whole army. Kalakeyas are unorganized savages controlled by their warlord Inkoshi. There is no command hierarchy in Kalakeyas to lead the sections of the army in battlefield. The commanders present in their army were shown to strictly follow the orders of the warlord. They plan their strategy well in advance and follow it during the war. So, it would be easy to destroy the Kalakeya army if the warlord is killed. In such a restricted condition, Trishula Vyuha comes to the rescue of Mahishmati.
The Trishula Vyuha is actually quoted in the Mahabharata. On the Eighth day of Kurukshetra, Bhishma adapts Kurma Vyuha for Kauravas and Pandavas come up with Trishula Vyuha to counter it. The geography of the battle ground should support Trishula Vyuha. In Mahishmati kingdom, the entrance is a narrow path covered by hills on both sides. The Trishula Vyuha (Trident formation) requires Three wings of the army with the central wing defending the narrow entrance into the kingdom while the Two wings on both sides march towards the Kalakeya warlord through attack. The major limitation of this strategy is time restriction as the delay in killing the warlord could result in Kalakeya army breaking into the kingdom. If the Kalakeya army manage to enter Mahishmati, then they wouldn’t wait for orders and destroy the kingdom. So, this strategy only works if all Three wings are highly efficient and work in good coordination otherwise it would make the work of the enemy easier by splitting the army.
Kalakeyas realize it at the last moment and try to stop Baahubali through a barrier which he breaks in. Baahubali and Bhallaladeva attack and kill Inkoshi before he could come up with a strategy to counter them. As soon as their warlord is killed, Kalakeyas lost hope and eventually ran off the battlefield or got killed.