QUOTE(ryansoh @ May 15 2016, 03:10 PM)
anyone knows which heroes the bios are the hardest to obtain? I know the A-force sister grimm, she hulk and singularity, then all the world bosses. Who else the bios are really hard to get? I really want sister grimm
I'm thinking to build a team with floki, ant-man, one more teammate I'm planning sister grimm, but her bios just too hard. bios I got from journey rewards not sure who to spend, even though I know which heroes I want. they just won't let me choose sister grimm because she is still new character. but ant-man I can get a lot from special missions, so I'm really not sure who to spend on. any suggestion?

"Hardest" character to get would be Carnage. Only because he's behind a paywall. And to get all his gears to lvl 20, you'll have to buy 2 monthly subscriptions. Unless you have a 5 star and 6 star rank up ticket, you may only need 1 monthly subscription to max him out.
Since you already have Loki, other good, rare characters without spending any money are (from rarest to easiest-ish):
1. Sister Grimm: Free daily bio selectors, rank up tickets, Battleworld
2. Silk: Free daily bio selectors, bio selectors x10/x100, rank up tickets, Battleworld
3. Ant Man: Free daily bio selectors, bio selectors x10/x100, rank up tickets, random ally shifter in special missions, lab bio converter/warp device
4. Hulkbuster: Free daily bio selectors, bio selectors x10/x100, rank up tickets, random ally/enemy shifter in missions, Battleworld, lab bio converter/warp device
I think Ant Man can be obtained from Battleworld as well. Not sure. Out of these 4 characters, I suggest choosing Silk (if you can) for your journey of growth quest thing. You can use all your energy and daily bio selectors on Ant Man.
There will also be a new daily log-in reward event coming soon too, so you'll get either a x300 bio selector or 6 star rank up ticket. You can use that on Ant Man or Hulkbuster. Hulkbuster will be a little easier to get through the free daily bio selectors, cos of the amount of combat characters in the game. Sister Grimm will take a long time to get if you don't spend money. If you really want her, I suggest saving a 6 star rank up ticket (if you get one/have one) for her, and slowly get 310 of her bios through the free daily bio selectors.