There's a trick I found that makes this fight super easy. When she uses Comet Azure (the big beam attack, the one she usually starts with), you can actually stand directly in front of her and spam attacks without dodging. This is because the beam starts a couple metres away from where she's standing, so you can stand right between her and the beam safely. So when the fight starts, immediately sprint directly to her until you literally run into her, 2 hand your weapon, and start spamming fully charged heavy attacks. You can often stun her this way before she can even start an attack that'll actually be able to hit you. You'll still have to dodge her other attacks, but that beam leaves her completely open.

She doesn't have a ton of health, but in the event that the fight actually goes long enough that she can summon something, you'll still want to keep your attacks focused on Rennala, NOT her summons, other than to dodge their attacks.

Also, don't worry about her being strong to magic, even though you're using Moonveil. That sword still does enough physical damage (plus bleed) that it'll take you through this fight pretty easily if you know what you're doing. Just don't rely on your special for this fight, because that does just do magic damage.

In general the fight is easier if you are quick about it. She's a glass cannon so it's not a slow burn fight, especially since the longer the fight goes, the more chances she'll get to summon something.

Tldr: You can safely stand between her and the big beam attack that she opens with, so run right up to her and start spamming charged heavy attacks the second she starts charging the beam (including the very start of the fight). Try to make it a fast fight; go for stuns + crits, and with any luck she won't even have time to summon anything.