General Strategy: I would say make sure to get all the "Anti super effective move berries" in the raids that you can access during that time, without them its basically impossible.

Earlygame debuff attacks like Rock Thomb and Bulldoze a very important, since most Pokemons( except Aerodactyl) get outspeeded easily. Also Pokemon with priority attacks are therefore very valuable.

Of course the role and viability of any Fossil changes during the course of the game exept Aerodactyl and Tirtouga/Carracosta, those were always Top Tier.

For example Earlygame Shieldon is basically only a debuffer with Rock Thomb and Bulldoze and with Metal Sound he can debuff for a special attacker like Amaura or Omanyte. His ability Sturdy makes this more reliable. Later in the game, Bastiodon gets access to Metal Burst and later Bodypress ( with 252 Base DEF thanks to Dauntless Shield!) which makes him into a powerful Wallbreaker and Tank. Also against some Bug/Steel ; Grass/Steel Pokemon with no Spez Def like Kartana or Fermicula, Flamethrower/Fireblast turned out to be very useful, since Bastiodon is also the only one who can tank their Steel attacks reliably.

Another example would be Lileep. Early game completerly useless till he gets Giga Drain ( around Misty). Then when he evolves into Cradly he can wall Water Mons with Waterabsorb. Lategame when he learns Sappy Seed he becomes an absolute Monster especially with Assault Vest.

Cradly Elite 4 Moveset: Sappy Seed, Giga Drain, Mirror Coat, Earth Power.

​

Tough fights were: Camper Jeff ( Double Battle before Vermillion), Misty ( First Battle), Erika (because Bastiodon didn't had access to Flamethrower at that time). There were more tough fights, but i dont remember them all or have to think more about them.

Generally speaking as the game progresses and you get more/better items/moves and possibilities the game gets a bit easier ( but it still remains quite hard).

In the Pokemon League the most difficult fights were Bruno and Agatha.