Chapter 12 has the No Turning Back, Where She Lives, Breath of the Glacian missions, as well as a battle against a Deathclaw. If you've stumbled upon this page early, then head back to Chapter 11.
No Turning Back, Where She Lived
Recommended level: 34
When the train enters the tunnel start running towards the front. You'll come up against a couple of batches of Snagas and Gargoyles, and shortly after the third wave begins the train will exit the tunnel and Leviathan will appear to save the day.
Follow Aranea, then speak to series regulars Biggs and Wedge. Cross the bridge and speak to Maria, then head back to the train, stopping to speak to Aranea on the way.
As you approach the station a tactless child will interrupt you, and there are a couple of valuables tucked away on the platform, you can talk to Ignis and Gladio, and listen in on a few convesations between passengers. When you're ready, board the train.
Into the Arctic Crevasse
Recommended level: 36
Wander up and down the train and pick up all the treasures and potions that someone's left lying around, and then just wait for a while.
Need more help? Our Final Fantasy 15 guide and walkthrough can provide tips on main story, as well as the open-world's many quests and activities. Learning how to get AP fast, EXP fast and money or Gil fast will aid you in many optional dungeons and tombs - including how to open and explore their locked doors. There's also plenty of interesting side-quests, too, such as the Scraps of Mystery and Professors Protege frog locations. And if you want to get around easier, you can rent a Chocobo, learn the infinite sprint trick and later in the game, get the flying car Regalia Type F. There's also more to see and do with DLC, such as Episode Gladiolus and Episode Prompto.
Breath of the Glacian, Deathclaw boss battle
Recommended level: 37
First you'll be up against a few Snagas and Wraiths which shouldn't provide much of a problem, but after the second wave you'll be facing off against Deathclaw.
His only weakness is Fire, and he has a number of powerful AoE attacks, but by making use of the Warp Points at either end of the area you should be able to wear him down whilst staying out of reach - he can't hit you while you're hanging, so if nothing else you can simply keep bouncing between them and him.
Once you're back on the train head along the corridor, follow the markers, and watch the exposition until you can get started with Chapter 13, which is a bit of a monster. Good luck!