In DragonBall Z: Xenoverse, Zeni is everything because it's the lifeblood of character customization. You need zeni to unlock not only outfits, but complimentary stat equipment and skills. It's important to have a ton of zeni on hand if you ever want to retool your character or change anything about it. Thankfully Zeni isn't too hard to come by and by and large you'll have lots of it after completing the story missions (which you'll want to do to unlock everything in the game).
The easiest way to obtain it is to fight Time Patrollers who drop Hercule Belts. These belts sell for a lot of Zeni and you probably already have a stack in your inventory. You can find Time Patrollers easily in a lot of missions, but the easiest one to defeat will be in parallel mission number 2. They sell for 15k Zeni each, which you would only need 3 of them to buy SSJ. You'll naturally get a ton of them over the course of battle.
Parallel quests give a Zeni reward, with the higher tier secret quests giving around 30k to just finish them and the lower level quests giving a thousand or so on quest complete. You can farm the later quests easily as your character powers up, so grinding through quests is another method to earn money quickly.
Finally, everyone's favorite parallel quest #53 awards somewhere around 25k zeni or more and requires little effort. Of course, by the time you unlock quest #53, you might have all the zeni you need.
Zeni is used to unlock skills, equipment, and to purchase consumables.
Another neat feature of DBZ:XV is that you can enter tutelage under one of the masters in DBZ like Vegeta or Krillen. You'll be approached by them in Toki Toki city and they'll offer some interesting quests for you to complete. As long as you get above D rank on the quests, you should pass, and this is how you unlock skills like Final Flash.
There is sometimes multiple ways to get skills, they can either drop from missions (often times it's assumed that if a character uses a skill, then it has a higher chance to drop), from mission triggers (perfecting a mission which requires a character to go SSJ/Kaioken/use a skill), from mastery quests (from various DBZ legends), or purchased in Toki Toki city.
Since most of the game is balanced as best as it can, the advantage of various skills isn't specifically they do way more damage, but they compliment various builds. If you've invested a lot of in attack damage and melee supers, then you'd want to unlock melee supers and they'd compliment your character the best.
Skill drops are random and even if you trigger an event in which they would drop, that doesn't guarantee that you'll get that skill. A lot of DBZ: XV is patience farming, grinding missions over and over hoping that the skill you want drops. Otherwise, choosing a premade character can be a faster route to using a skill / build that you want the most.
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