During Driver’s License Lunacy, upon taking control of Vegeta later on, you can exit Capsule Corporation and see that this Substory is available to you. Northwest of Capsule Corp, on a cliff not too far from the lighthouse, you will find Tien. Speak to him to find out that he is doubting his power, as some people seem to be racing past him in terms of battle power. Vegeta, being himself, insults Tien and that gets him to fight you.

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Tri-Beam leaves Tien wide open to beam attacks

If you remember the fight from the Saiyan Saga with Tien, when you were controlling Piccolo, then this fight isn’t anything different. Tien only has two main attacks, Dodon Ray and Tri-Beam, both of which are pretty easy to dodge. You could probably counter Tri-Beam with a Galick Gun or something, since it’s a bit slower to come out than the Dodon Ray. The substory will be over once you win the battle.