Vortex Arena is basically Arena but with some special rules. One of them being, you can only use the featured element for your units.

 

The Element Cycles as follows: Fire > Water > Earth > Thunder > Light > Dark

 

You bring in your strongest Mono Element team of the element featured that month and you brawl it out with others. If you beat an opponent, you get Tournament Points or TP. After you get a certain amount of TP you rank up. You get rank up rewards based on what rank you achieved. Another set of rewards you can earn is from the Event Ranking. At the end of the event, depending on how high you score, you are ranked and you get additional Seasonal rewards based on it. 

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As of December 18th 2019, Gumi introduced rules that stay in effect for the event's duration. Rules range from: LS Disabled, BB Disabled, SBB Disabled, Sphere Disabled, etc.

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With these new rules, you have to be more mindful of your squad building and spheres equipped. As a beginner, I don't recommend trying to rank high for event ranking if you can't afford it. It would be best to just get the rank rewards if you can.