Enter the Arena Immediately
Unlike the Court of Oryx, the new Archon’s Forge arena in Destiny: Rise of Iron requires you to enter the area immediately after you make an offering. Once you’ve entered a SIVA Offering in the Fallen terminal next to the arena, make sure to get into the pit once it’s processed. If you dawdle, you will be locked out of the challenge, and you’ll have to use a Splicer Key to get back in, which is a waste of items.
So here’s the first pro tip to keep in mind: get into the pit immediately after making your Offering just to be safe.