Hey guys,
I discovered this method by mistake, and I thought I'd share it here, in case some of you don't know it.
Anyway with this method you can farm unlimited Nethershards for as long as the invasion is up.
Basically it goes like this:
1- Go into the zone with the invasion and just do stage 1 normally.
2- In stage 2, make sure to hit the lesser demon commanders in the town once to "tag" em, and you'll get Nethershards upon their deaths.
The demon commander in stage 2 drops no Nethershards.
3- In stage 3, look for SKULLS on your world map, these are mini-bosses that everyone will be racing to kill them, make sure to hit them once and stay back, each one of them drops 5 to 8 Nethershards upon their death if you have hit them.
4- Wait until stage 3 ends and stage 4 begins, that way you will get 10 Nethershards from stage completion, then go out of the zone completely until the whole invasion thingy disappears.
5- Go back into the zone, and you should have a new scenario from stage 1, with new people. Rinse and repeat for as long as you like.
This allows you to farm Nethershards as much as you want for as long as the invasion is up.
Enjoy!