Fodder off an extra Dorcas?

I highly recommend you fodder Dorcas for Infantry Pulse 3 rather than QR. It is a much more exclusive skill compared to one that can be found in the 3-4* pool.

For gameplay purposes, QR2 is as good as QR3 outside of high tier arena scoring purposes. Only fodder Dorcas for QR if you plan on never EVER building an infantry team (this includes something like an infantry dragon team) or running a build that would work with the cooldowns (e.g. s!y!tiki w/3 infantry dragons)

Hero merit farming is finding a good autobattle solution to the Hard/Lunatic maps with Level 40 units by unequipping weapons/assist skills from all units who do not engage in combat.

Here's a quick guide of pre-built solutions, although one of the joys of farming is figuring them out for yourself (note that sometimes you need to adjust your strategy to compensate for unit stats): https://docs.google.com/spreadsheets/d/1ALxzhV7pHoOr9dV15NiimBAP5qgdzQH274FUBJk5LlQ/edit?usp=sharing

You can actually build a Dorcas to deal with mages. He has a superboon in Res making that 28 Res. IMO, I think you should keep him, but the choice really is yours, since you already have a +Def Dorcas you can fodder him for Infantry pulse not QR as its Available in the 3/4* pool.

+res with distant def seal helps him tank mages. He's busted against any dragons that aren't -spd unless they have approximately 4+ merges, in which case he's still busted.

+def just makes him extremely tough, especially if you give him steady breath.

I have -spd +def, and I have to say that it's quite scary to tank a green mage even if he had +res and distant def. That gives him 34 res and 45HP, which means they need to hit twice for 57 if they don't fire a special. If Dorcas can get a res buff, I am more convinced. Who will hit twice and not activate a special? Blade tomes or non-optimally built NPCs such as what we see in TT. Maybe Julia/Deirdre because they're using glacies. Otherwise, I'd expect a moonbow or a glimmer or even an iceberg (Merric's cooldown-acclerating prf or now that we have flashing blade seal). So tanking green mages with Dorcas isn't easy. As for blue mages, they tend to fall in one hit if he's using fierce stance. The deadly ones are a squishy bunch - Delthea, Linde, Ishtar, Mae, Tailtiu...Reinhardt, I leave for you to figure out. Quickened moonbow creates some interesting issues, but you do cut his attack by 20% using weapon triangle advantage.