First off you need at least 2 of the same unit. Let's use my double Rein for example. The more of a character you have, the better this'll be, but I don't have 4 fully trained Rein laying around so we'll have to use these 2.

My fully trained +HP -res Rein

My wip +def -res Rein

Next off go to the training tower. LOOK AT THE PEOPLE IN THERE

For the 10th, its 2 Red and 2 Blue. I ditched Berkut in favor of my +atk -res Julia. Now go into a match

Now, if one of your units isn't trained, KEEP THEM OUT OF DANGER! The point of this is to keep both of them alive. So here, id keep my 17 HP rein out of the danger zone. Do the match like normal now

(I know it isn't the same map soosh. When I tried to get if the first time for this it didn't work)
Now you see that the Reins each have their own hero merit gain. I got about 6.55 hero merit from that one battle. If I had 4 Rein then I could've gotten a lot more hero merit, but I'm f2p and I'm lucky I have 2 rein. Now say if you had 4 Hinata, then you could do the lower levels, grind off of that till you hit the cap of 3000 Hero merit. Per unit you get 3000 feathers so times that by how ever many units you have dupes of and you have lots of feathers for getting your units trained or si.

Yes I did all of the math for this tutorial.