The Magic Training Arena can be found behind the dueling arena. To get there simply use a Ring of Dueling to teleport to Duel Arena and then go behind it.
Invest a chunk of cash into runes, don't worry you will make profit never the less. The goal here is to be able to buy full infinity using your "MTA" points.
Go to the grand exchange and buy around 12000 Law Runes, around 15, 200 Nature Runes, and around 270, 000 Cosmic Runes. This might seem like a lot but you will make profit and experience. You also might want to invest in some elemental staffs or else you will need the second runes to cast some of the spells.
Travel to the "MTA". There are rooms where you perform a certain task. You want to use your Cosmic Runes in the Enchanting room. Always enchant with your highest enchant spell for maximum experience.
Go to the Alchemy room. You will see what looks like bank deposits but in fact you withdraw stuff from them. It's recommended withdrawing some from every stall. Once you have a good amount of stuff you will see a thing at the bottom of your screen that will say an item. You should alch this item as it gives you the most points, and points are very important to the money making process. You should use up about half of your nature runes here until you have enough points.
Go to the graveyard room. In this room you take some damage but the object is to pick up bones and turn them into bananas. You can eat the bananas to heal but to get points you must deposit the bananas in the middle. When all your natures are gone you should have enough points from this room.
The last room is the telegrab room. In this room there is a statue in a maze and you must use your telegrab spell to move it to the middle. Moving it to the middle gives you points.
Once all your runes are gone you should have enough points to buy Full Infinity. After all is done and you sold your infinity you should have made a couple million gp and gained a hefty amount of experience.