How do I get blues above 340? All my engrave seem to decrypt to that. Sitting at 348 light and only Legendary seem to give small light boosts.
Some people have mentioned, but yeah, regular day to day blues will only go to 340 max. If you get them as drops from heroic strikes (and currently the director 320 version of a few, the newer ones), Nightfall and CoE they will go higher. Otherwise, only legendaries and exotics will push past 340.
I also believe LL growth is sort of gated by decades, so if you're 340-350 you won't get a whole lot that goes past 350; but when you get all your gear up to 350, then you break through to the next set.
It sounds (and kind of is) convoluted but I can't really get upset that I'm not at max light less than 72 hours after the expansion went live. I'm not interested in grinding like crazy like that; I'm at 345 currently and still picking my way through.
Re: Archon's Forge - I actually like it a decent amount but I must have incredible luck because the few times I've run through I've stumbled into random 3+ man groups already there. I was running a patrol and just passing through when I decided to jump in - then during the round I got a perfected siva drop and everyone else started dancing like crazy! They went ham on shooting the console for mr to drop it in; then after the round, they all got rad loot and I think I only got a repeat legendary.