Congrats mate.
What strat/weapons did you use to beat Costlemark? It still kills me that I couldn't do it.
I'll try one more time when I get home next week.
Noctis loadout:
Sword of the Father (equipped but never used, passive +100 Str/Mag)
Shield of the Just (super tank Noctis, allows for no one shots with 500+ def, the -100 Str isn't a big deal)
Zwill Crossblades (Best daggers/ground DPS weapon in the game, this is all I use unless I'm doing aerial combat)
Bow of the Wise/Healcast (Alternate between the two. Healcast for when Noctis is under 50% max HP, otherwise Bow of the Wise for Overwhelm spam)
Accessories:
Noctis - Ribbon, x2 STR Accessories
Gladio - Safety Bit, x2 DEF Accessories
Ignis - Safety Bit, HP/DEF Acc, HP Acc
Prompto - Safety Bit, HP/Def Acc, HP Acc
(Final boss of the dungeon, I swapped out the Ribbon/Safety Bits for Fire Res).
With this, Noctis and Gladio had well above 550+ Def and Ignis/Prompto at 350+. Noctis had ~6k HP with the rest of the crew at 9k+. Noctis needs Safety Bit/Ribbon, otherwise the run is over with one stray IKO attack from an Elder Coeurl. Otherwise the rest of the party is just there to survive and to let me use their techniques. Speaking of...
Techniques:
Gladio - Impulse
Ignis - Regroup/Enchantment/Overwhelm
Prompto - Piercer
Regroup was the most used during trash fights, with Impulse if the fight somehow dragged on. Piercer + Enchantment > Overwhelm combo (with Bow of the Wise equippped) shreds all the mini bosses and last boss. Just make sure Noctis is at full HP.