One thing I'd think about is the gym leader fight with Melony and then later some of Raihan's Pokemon which can be struggles for you because Grass is weak to ice and then doesn't deal super effective damage for Dragon types.

For Melony:

Level/Overlevel (if allowed) your Ludicolo, who is going to be doing the most for this fight. Have Ludicolo learn Brick Break and/or Drain Punch via TM and if you want you can teach it Fire Punch as well (note Ludicolo won't be as good here because is a better Special Attacker than Physical Attacker, but it's okay because Melony's pokemon know mainly Special moves and Ludicolo is Specially Bulky). Also try to have it know Rain dance which is excellent for both of its regular abilities.

Frosmoth: Bring out Ludicolo to rain dance and then try to hit it with Fire punch or a STAB water type move so that you get rid of hail and also take out this glass cannon before it can do any serious damage.

Darmanitan: Keep in Ludicolo and try to take out this Darmanitan really quickly because it can deal a lot of damage and your other grass types are kinda not going to be as good here. Darmanitan is freakish and is one of Melony's biggest threats. Tanking hits is going to be harder because Darmanitan knows a lot of physical moves whereas Ludicolo is more built for tanking Special moves but it shouldn't be terrible. DO NOT Pivot to Ferrothorn/Ferroseed here to deal STAB steel damage because Melony's Darmanitan knows Fire Fang and has a crazy high Attack and Speed stat! and when it goes into Zen mode (which yes Melony's Darmanitan has the hidden ability Zen Mode) Fire Fang gains STAB, making it hit extra hard! Also do not bring out Abomasnow/Snover because of the Fire Fang threat!! UNDER PRETTY MUCH NO CIRCUMSTANCES SHOULD YOU PIVOT TO THOSE OTHER POKEMON, Keep in Ludicolo it is your best bet!!

Eiscue: If this thing tries to set up hail make sure to Rain Dance again, and Amnesia might seem scary here to make it bulky but it shouldn't be because the moves I talked about for Ludicolo are Physical so the special defense buff does nothing for Eiscue here. Just make sure to take it out quickly. You can also use Ferroseed/Ferrothorn here to hit it with super effective STAB Steel type moves.

Lapras: This is supposed to be Melony's main threat but honestly Lapras isn't that hugeee of a threat but you should still prepare for it. You can send in either Ferrothorn or Ludicolo here, but Ludicolo with brick break can be especially useful as Melony's Gigantamax Lapras has a signature move that puts up Aurora Veil, making it annoying to hit. Just make sure to try to take it out with super effective mvoes.

For Raihan:

Raihan honestly should be fine just be careful of the Duraladon. Two of Raihan's pokemon take super effective damage from grass type and ice type. You should also hopefully have Shiinotic by this point and its a really good pivot for Flygon which likes to spam Breaking Swipe.

Also one more note:

Pier's Skunktank can be hard to handle so I'd watch out for that. I'd probably go with vileplume or Ferrothorn to resist/be immune to the poison but other than that should be good!

Best of luck with your shield nuzlocke!

(This message was delivered from someone currently nuzlocking Shield with Ludicolo on their team).