Day 1
Since Guardian has the highest health pool in the game, try your best to stick with your team and use your pins effectively. Pathing around the map is extremely important to farm more efficiently and make boss fights much easier.
Your priorities for Day 1 are to farm as many locations as possible to maximize your level and find a usable weapon and shield.
Observe the map and take note of locations with bleed, rot, frost, and holy symbols, as these will guarantee a weapon of that affinity as a drop from the area boss.
Prioritize bleed and rot locations as these will do the most damage to Gladius.
Also, use the overview feature on the map to guarantee useful weapon types from locations.
Now start making your way to those locations, stopping at as many churches and smaller settlements as you can manage without wasting too much time and getting caught in the storm.
Equip any weapon you find with a useful passive.
Day 2
On day 2, many powerful field bosses with rare drops will begin spawning.
By now, you should have obtained a suitable weapon and the levels to challenge field bosses for extra runes and loot.
Now lead your team towards those field bosses, stopping at any churches and small encampments along the way.
If the field boss looks too difficult for your group to handle, you can always skip it and go to the next one.
There is a good chance you will find something more manageable at another location, and it's a lot better than dying and losing levels.
Now you’ll need to sift through your weapons for the best passives you can get.
Day 3
Take your time here.
Make sure you level up first, and then purchase any necessary equipment from the merchant and upgrade your weapons of choice.
Get rid of any extra upgrade materials or keys in favor of consumables, as you will not be able to use them during your fight.
BUY
Warming Stone!!! This will grant your team another source of reliable healing, which you will find is a scarce resource during these long fights.
If you have a mage and some extra runes, get them some
Starlight Shards, as this will significantly increase their FP pool for the fight.