So I am 275k might with 5 heroes at BT Lv 10+. Should I start focus it now? Or save the resource for newer heroes?
Well, I've barely scraped the surface myself, but as I see it, it all depends on what your account looks like. I like to hoard my books for other potential purposes, and fail to see a reason to use them on breakthrough. I'd much prefer to sweep, and therefore, breakthrough leveling goes much slower. I suppose by "focus," you mean put a lot of resources into that. So I probably won't ever do so. Sweeping is fine for me. Another thing to take into consideration is the levels of your heroes. As you're at 275k might, I'd assume that most of your best heroes are devo. So the next question is, do you focus on one hero at a time, or a few elite, or all of your best heroes? I'd go with one at a time, as the buff becomes substantially better after about BT20. 1 BT20, in my opinion, is better than 4 BT7's. So, to answer your question, "when to focus on Breakthrough," obviously, you'd start pouring all of your exp into breakthrough once all of your necessary heroes are at EE 200. I wouldn't wait till then to start, though, as it seems you haven't.
Which heroes we are talking about ?? And how many res you have ? For now i would hold on cause we don't know how new hero will look like, and hot it will be available. If through gems, and it will be strong one, than you can dump it to him, if it will be new dragon, which will not be available through rolling, than we will think who to pick from those 5 (unless you will be willing to buy that new hero)
Do a combination of both, IMO. Sweep groups of 6, and daily before you log off, top off their BTlvl. But obviously focus on the ones that you use the most. That way you utilize sweeps plus resources. . It's a slower ascent, but definitely easier on your stash. It does a lot to know that you're bumping them up daily as well. That way you don't feel like you re just stuck in a rut. If you get antsy, top off to 20 your main ones. Then slow roll them from there.again, just my opinion.
I prefer to focus on a single Hero up to level 14 or 15 Breakthrough, and then move on the the next (that's when I've found the bonuses are fairly significant). I'd start with Lava or Dove, depending on who you have. This is also just my personal preference. I like having a one or two Heroes powerful enough to carry my team, rather than multiple Heroes that are slightly above average. If you're planning on buying the new Event Only Hero, then save the resources for that. Otherwise, I'd start working on your most important Hero.