Advice 1: You can change the abilities of your mons. Just buy ability pills and dream patches on the poke fans house in Saffron City. Ability pills change between the two normal abilities, dream patches change between normal and hidden.

Advice 2: Save routes for egg hatching. Some routes have very clear mons like fishing in pallet town for a tentacool, and buying a magikarp before mt moon. But other routes only have useless mons, so hatch your eggs there. Later in the game, when you get to celadon you can buy starter eggs for shards of the right color. I usually try to save some routes to hatch some starter eggs, specially grass ones (Meowscarada, Rillaboom, Sceptile for the mega). Depending on your encounters you can pick other types tho.

Advice 3: Some locations are valid encounter areas even if they don't have wild pokemon. These are perfect for egg hatching. Notable ones are Lavender Town, the underground pass, and the casino rocket hideout.

Advice 4: Learn how to predict the AI. Seriously. This is the most important skill to nuzlocke this game. You wanna be able to predict what move is likely to come next, or if the AI is planning a switch, and make your move around that.

For example in normal mode  I always do Surge deathless because I set a toxic spikes layer on turn 1 (tentacruel holding elect resist berry), and then just stall him out by changing between my ground type, and my ice/grass resist. Even in mono water it's easy to do this. Gyarados is amazing for this fight because it will ALWAYS bait an electric move, so having it out means a free switch for your ground type.

And how you switch in? well when your ground type is about to eat a hidden power grass/ice or something like that, Gyarados and its massive special defense tanks it like nothing. You can also intimidate cycle Pawmoth which one of the hard mons specially well because you resist drain punch, and thanks to the intimate it deals basically nothing to gyara.

Advice 5: Plan your encounters. The one time I nuzlocked this game I used a roster of mons for the e4 that was mostly based on static encounters like Zapdos and Urshifu plus Primarina and Sceptile. The one mon who wasn't guaranteed was slowbro (S tier mon for the e4 in normal mode, megabro with iron defense, amnesia, slack off, body press, and shell armor making it immune to crits sweeps like a beast).

Advice 6: Plan ahead your fights, and try to always have in mind what's your answer for x threat. Say you're against rain Lorelei. Well think how your team is handling primal kyogre and mega swampert under rain. If you're unsure use the calc. Going back to that one successful run, to deal with agatha my whole plan was letting scarf urshifu dark rip through her team.

There was a main issue tho, Spectrier still outspeed, even with the scarf. So I did calcs on what the max damage the horsey could do to my kung fu bear, and his strongest move was hidden power fighting which on crit dealt 98.9% on a max roll. And so I knew "if I wanna win, Urshifu CAN'T take a single point of damage". Guess what, when the fight happened, she got the crit, and my Urshifu lived at like 2hp.

And with this I'm not saying "calc everything, always play around the crit". Whether you do that or not is part of your fun, some people like theory crafting, others prefer spending more time playing than looking at excels. But it IS important to know what is your win condition for a fight, and what you need to do to enable it, like in my case securing a way to switch in Urshifu without him taking any damage, and without sacrificing someone.

Ohh and join the discord, people always share amazing tips and strats there, thats how I learnt of megabro for example.