It really depends on where your PvP store heroes are at currently with stars. Halo is a great option since she’s strong in both PvP and PvE - plus she was just added to the store so it’s very unlikely she’ll be removed for a long time.
don’t you think choosing a hero which is in both the store and the campaign (9 frags a day guaranteed) to be faster? but sure, she’s great
Well you already have Mandrake at 10, so you can at least already max out your free quick wins on hard mode frags.
I don’t personally run hard mode with stamina ever - I do my 9/day from watching ads, but that’s it.