Are you guys feeding or burn?

There can be a lot of damage going out in that phase, use cooldowns, convalescence is especially nice at the start. Kite the slugs around a bit while spamming flash as healers adjust to all the aoe damage and get you topped. Make sure you are not hitting the slimes with flash, they will get pissed and hit you hard. If you are doing feed method then feel free to use big cooldowns like sentinel right away, hallowed is all you need for the giant slug.

If you are doing burn I'd highly suggest just using mage LB and killing first set of slugs while doing some damage to boss. If you can't manage to kill with burn without melee LB3 then you're just going to have to rotate cooldowns the entire time and hope your healers can keep you up. There's not a real trick to it.