The Dinosaur Forest can be found by going north of the Veldt to the old man’s house, then east to a forest. Inside is the main focus of our journey here.
The Brachiosaur will easily shatter you. For quickies, it can use Meteor and Ultima as NORMAL spells; as in, it’ll use them repeatedly if it wants to. Oh, and it wants to. Given that the Ultima is at the level of the Magic Master’s – 6,000+ damage to all – you’ll want to be careful.
There was only one suitable method to this I could figure out that did not involve intense grinding without Diabolos – I doubt you have higher than 3,500 unboosted HP on anyone. Anywho, I’d save before trying to fight and, during the battle, I’d use Ragnarok’s Metamorph ASAP. Given that this thing has a hit rate easily around 50%… Come on. Just do it.