How To Getting to Thundoria

You have to walk all the way ....
Just keep to the coast and avoid the higher lvl mobs.
I walked from Valhalla haven with a lvl 35 explorer and made it alive. I walked north to the coastline and followed it north, had to walk around some mobs but i made it.
Chaldea haven is surrounded by high lvl mobs so I wouldn't recommend leaving from there. 

 from Rewl

Or you can go to Chaldea Haven, run straight to the north. Be ready to have lots of healing pots and try to avoid mobs. Just follow the road and you'll get there. 

 from lerie

Thanks for the pic Yotup!

You can get to thundoria by using thundoria ticket from fortune/prosperous packets now.

from TwilightHikari

Or you could buy a Thundoria Ticket from the Item Mall, another player (usually sold from 70K-100K each), or if you are really lucky from an NSS (though the chances are very slim).

Those were ways suggested by many players, but here is a way that allowed me to get to Thundoria Castle at level 15 and a Guppy (and no HP pots but I really don't advise that). It's best to have many HP pots just in case so you don't suddenly die or anything like that.

Here is what the path from Thundoria Harbor to Thundoria Castle and the monsters nearby look like. The red dots aren't their real place it's just my approximation of where they are while attempting to stay alive. I must say before anyone says anything that I MISSED THE TREANT in my little diagram since I didn't want to re-do it and at the time I was too busy running for my dear newbie life to register that a Treant was there.

At lvl 15 and 5 for all of my stats (I actually forgot to up them before I left the safe area, oops) I managed not to die in just one shot but two shots (wow what a difference...). So, with a Guppy, I traveled from Argent City to Thundoria Harbor (which took a while) and cost approximately 200 fuel out of my 500 fuel at max.

Put in your radar (remember to select the Ascaron region) 1096, 1336, which is the approximate location of Thundoria Harbor, and set off to where your radar points to, avoiding the sea monsters on the way.

When you get there, head out of the south part of the Harbor. You WILL meet and get attacked by either a Guardian Angel (slow moving, mid-range attack I believe) or a Spiky Stramonium (stationary, long-range attack) so get your Cakes or other HP pots, put them onto your Hotkey, and run without looking back while healing yourself until you get to the Safe Spot.

Follow the Black line and stay away from Red Dots and you'll be on your merry, safe way to Thundoria Castle. Be sure to save your spawn point there so if you want to head back to the Harbor in the case that you die, you won't have to buy another ship.

I had some help from LostGirl619's diagram in making my diagram with the monster locations.

TIP: Save your Spawn Point at Thundoria Castle so that if you want to go there you could just buy an Old Ticket for ?500 (or kill Smugglers, your choice) to get there easily (and save any other lost ships at the Harbor close by).