How to improve my hero collection with 2.0 boxes?

Hey guys!

Currently I have 27 heroes and I want to expand my hero collection as much as possible. I don't care about skins, so what I'm really looking for on my loot chests are heroes.

What's the best strategy to get the highest amount of heroes? Should I get the mega bundle first and than open the boxes (getting shards for duplicates) or opening the chests first (and then choose the bundle with the least amount of heroes I already have )? All chests rarity are worth rerolling?

Thanks in advance!