Most events like this where there are multiple mobs spawning require you to think about endurance. There's rarely enough time to regain spent resources which can leave you vulnerable to the "boss" mob at the end. There are plenty of quest fights that work like this, including some truly nasty fights during the Main Questline.

I struggled with this in my no-CP level 15 warden, but eventually prevailed. Without knowing your class, I can't give class specific advice, so I will try to give general advice applicable to most classes.

Resource Management: having the resources to use your skills is super important. If you are running out of your main resource, try to weave in full heavy attacks to gain back some of your resources. Weapons restore stamina and staves restore magicka. Having food/drink and potions will help with resource management. Food increases certain stats, drink increases regeneration of stats, and there are level specific and holiday-leveled foods that can be easily obtained from guild stores.

Skills: like EleonoraCrendraven said, equip a self-heal. All classes have one, some have more than others. After that, look at skills that do AOE (area of effect) damage. This is a multi-mob fight, so skills that damage multiple enemies will help reduce the amount of skills you have to cast. Finally, look at your skills that you can "spam" for more damage. These are usually single-target, lower-cost spells that you can use again and again for good amounts of damage. They may or may not be DOTs (damage over time). If you class has them, you could also use a lockdown skill if you need a breather from a certain mob.

Ultimate: unless your ultimate recharges quickly, try to save it for the rounds when you get into trouble. Its best if you can save it for the last spawn which is often the hardest.

Tactics: There are two many methods I use. 1. Stand and take it, while the mobs stand in my AOEs and DOTs. This works pretty well for a tank. It doesn't work very well on my no-CP level 15 Warden. 2. Kite the mobs in a tight circle, so that they are still in my AOEs while having to chase me. This works pretty well on my warden. With both methods, I begin by casting my AOE skill, add in a lockdown if needed, and then start circling or fighting the mob with my spammable damage skill, mixing in heavy attacks if resources are low or self-heals if I need them. The goal is for the mobs to take constant damage from my AOE without me having to cast my spammable damage skill a ton of times which would cost a lot of resources. The AOE needs to be refreshed once it ends.

I hope this helps. Finally, I'll repeat ZOS' advice in the sage words of the loading screens. Sometimes, if a fight is too hard, leave and come back after you've unlocked better skills, better gear, and have more experience.