It's very important to break Titan's ATK/MAG and buff your DEF/SPR.
Weight of the Land, one of the two magic damage it can use, is less frequent before the core is destroyed. So prioritize ATK break and DEF buff before the core is destroyed, but make sure to keep all 4 once you break its core.
All of Titan's attacks are non-elemental. Earth resistance is not needed.
Titan's HP cannot drop below 60% on the turn it reaches that threshold.
Titan's Core appears once Titan's reaches 60% HP threshold. He'll become invincible until the core is destroyed.
Target Titan's Core manually to attack, but make sure to keep applying break on Titan, and not the core.
The core itself does not attack until 5 turns after it's summoned, on which it will use an extremely strong variant of Earthen Fury, likely wiping your party.
It'll ramp up damage significantly after the core is destroyed.
You can avoid Landslide by triggering its threshold every four turns: