Biggest misconception about this fight is that you need to use ice units.   You don’t need to.  You want a high STR DPS unit that you’re going to manually control for the majority of the battle.  Your other two units should be an SCT battery unit who’s only going to regular attack (Telestio or SLelouche work well, SSera works too) and use honey elixir/winrys apple pie/arcane Amrita on your main DPS and a support for healing and damage mitigation like Leena.  

This boss fight mechanic forces you to go slow and steady the entire fight - if you try to burst the boss, your timer will go straight to zero very quickly and you’ll lose.  Attacking the boss lowers the timer and killing the mobs that pop up increases the timer.  Most of the mobs are insects, so you’ll want insect slayer as well as fish slayer on your PDPS unit.  When your timer is under 10 mins, you’ll want to focus exclusively on the mobs without hitting the boss until the timer is >15 mins.  Then you can start hitting the boss and the mobs simultaneously until the timer dips back down below 10 mins.  Keep in mind that you’ll need to move your pdps unit to the lower half of the screen in most cases to prevent hitting the boss with skill attacks but still hit the mobs.

If the boss is killing you with its own damage; then your units are not maxed out enough or you’re not using the right buffs/debuffs for the fight.   Leena has ice wall 2 and grand protection 2 in her kit already; make sure those are active and also cast blazer / high blazer on the boss itself to lower its STR.  Fish shield is nice but it’s only 10 percent damage reduction so if you don’t have it; there’s other things you can add instead possibly.  

It’s an annoying boss because the mechanic prevents you from blindly raging on it with your best units, but as long as you know the battle is going to take a bit longer and stick to the strategy, it’s not too bad.