Well for this gambit event the 3M is pretty lofty...I usually have all the slayers and I don't think I have ever made it to the last reward...if I have a lot of items I try and rank within the top 100 and I still don't get to 3M, so yeah you'd need to spend real $$ to get there. You might be able to do it if you had lots of stuff saved up and blew all of it, but it's not worth imho. I would also suggest waiting until the 2nd halve before using your items, when more slayers are available.

I've never tried this but I'm going to wait until the 2nd halve to open my briefcases since they made "raw" items only last a day.