In Ilex Forest at Night time.
Route 24, 25, and Ilex Forest at Night time.
It doesn't matter when you catch them.
oddish you have to get him at night time
Catch a Bagon and level it up, find one at crystal crossing
The only way to get a Mewtwo in Pokmeon Crystal is to trade with a copy of Red, Blue, or Yellow through the Time Machine in Pokemon Centers.
No Swellow is A 4th Generation Pokemon Crystal is only a 2nd Generation Game, and the time machine only grabs Pokemon from the previous Generation so there for you can not catch it since it does not exist yet and Crystal does not have a revisit like Gold and Silver.
Between 6pm and 3am in the ice path or mt silver
Every time you see a Pokemon that you haven't caught (if it has a little pokeball symbol by its name then you caught it already) then catch it right away. Then once the game it beaten look on your pokedex for the ones that you haven't caught. POKEMON MASTER * INFINITY
You can catch Tailow in Pokemon Liquid Crystal by heading to Route 32, where it has a 10% chance of appearing in the grass during the day. Use Repel to avoid encounters with other Pokemon and increase your chances of finding Tailow. It may take some time, so be patient and keep searching until you encounter one.
There is nowhere the player can capture a wild Bulbasaur in Pokemon Crystal. It can be traded using the Time Capsule.
It's up to you I wouldn't recoment it, but it'll be a decent grass type for the time being