According to Saldain's lootpool, he's got 8 weapons in the current rotation which is half of the triumph right there, though I see your point on being unable to get it for the season if you're a recent player. Looking at all of the IB weapon's we've had in D2, there's a total of 26 if my memory's correct, and a few are still Y1 versions that haven't been updated yet which they might eventually bring up to current standard in following seasons. If they rotate/add at least 4 weapons a season, you'll have it before Lightfall and that's assuming that Xur doesn't sell any of these to further help with acquiring them. Edit: I almost forgot, but Point of the Stag should also count towards the triumph and you can acquire it from the exotic kiosk if you haven't already.