Who to +10?

Atm, Soren

inb4 wait for 3.0 before merging

dont mind me just posting propaganda

Red - Hana/Soleil are good units due to being fast and being able to hit hard. Soleil has more attack and defense I think (have to check) but Hana is available as a 3* while Soleil is a 4*/5* unit. You can also go with Selena. She's one of the best mixed tanks for Infantry sword units if you have the resources. Male Morgan is a nice choice for mages since he can run a more supportive role with his tome. There's also Roy, Marth, Masked Marth, Draug, etc.

Blue - Donnel is a good choice and was the first unit I max merged. He's fairly flexible and can work with pretty much any IV set. Being a Lance unit gives him access to Berkut's Lance, so he's able to tank mages pretty well, and his high HP let's him comfortably run Panic Ploy for those annoying visible buff-relying units. Other good choices are Oboro, Nowi, Gwendolyn, Effie, and a few others.

Green - Hawkeye (who was essentially powercrept by Helbendi) is a really good mixed tank. Libra is another one, although they have somewhat different play styles. There's also Sheena (arguably the best mixed tank in game), Bartre, Barst, Arthur, and a few others.

Armoured units even the gen 1 BST ones are still great, because for their BST, which should allow you to get and stay in arena 20 no prob.
Sheena basically is one big fat wall that kills everything when her ignis/glacies activates.
Wendy is similar to her.
Both are obtainable in 3 to 4 stars. They’re not the best but that’s what you get with f2p units.

If you want someone GOOD and don’t care much about BST, get LILINA and kill your brave celica for death blow 4 (just kidding, death blow 3 would be good)
Her refine makes even Wrys shit his pants because of how insane her attack can reach coupled up with chill res.

I'd recommend the same ones I'm trying to merge for arena:
- M!Corrin because his refined Yato can give +4 to all stats
- Titania (with +def) for her triple Tactic
- Shenna (with +spd) for her BST and double ploy

Soren with G duel infantry can cover the same role as Sheena as debuffer but a bit better while scoring higher but this way there would be 2 infantries in the core.

Edit: now that I think about it instead of Sheena you can use LA!Elwood with a res refine for debuffs once you have enougth grails, he has 175 BST and is a "free" unit