The masterball.
arceus will beat him easily.
using poke assist
The easiest way to get one is to win the Bug Catching Contest in the National Park after acquiring the National Pokedex, but I'm sure there are other ways-e.g. As a held item by a wild pokemon, pickup, hidden item on a route etc.
The easiest way to get one is to trade a Krabby to a fisherman in Olivine City.
The easiest way to "see" a Feraligatr is to battle your Rival only if you chose Cyndaquil as your Starter.
i'd suggest that you'd battle more trainers. If you beat they're Pokemon you'll get a little boost in the xp than wild Pokemon battle
If you are talking about catching them, you cant unfortunately. I tried. but you get them as a starter Pokemon in the johto region.
there is no easy way im not exactly sure of the figures but there is an extremely low chance of catching a shiny pokemon, so just keep on hunting for it
You can catch a grass type Pokemon but catching mankey is the easiest because it appears a lot especially to the east of Pewter (where Brock is). Mankey has the advantage of being a fighting Pokemon which is super-effective against Brock's Pokemon.
I just caught Girantina in an Ultra ball. So, maybe an Ultra ball is the best for catching Girantinas.
With honey. Artixthepaladin
There is not really a easiest pokemon to evolve...but bug type pokemon evolve really fast