I have the TM toxic and my main goal is to gain the most exp. Question 1: Should I give the move toxic to to my lv 4k Articuno or my level 500 golurk. Question 2:What is the next thing I should do? I think its to learn fly and the only way 2 do that is to beat Zapmulcuno. Question 3: How to beat Zapmulcuno. Give me SPECIFIC instructions (I don't need 2 mess up). Also if possible, please check your method on Zapmulcuno just to make sure. Please also add who should battle it (with the pokemons I have). Question 4 : What do I do next. Keep in mind that my goal is to get the most EXP (for either golurk or Articuno). I think what I need to do next is to face the Mysterious trainer. Question 5:How do I beat the mysterious trainer (make sure you are specific on what move I should use, pokemons, etc.) IF there is anything else I should know, PLease tell. By the Way, I have 3 lv 5 golurks and 1 lv 500 golurk.

1.)You should use the TM toxic to your main Pokemon.
2.)Other people can teach your Pokemon the move fly for 50 coins or for free. Check some threads.
3.)https://pokemoncreed.net/forums/viewthread.php?id=156858-->; a thread on how to defeat Zapmolcuno.(I hope it will help you)
4.)Yeah! Mysterious Trainer will give you a lot of EXP.
5.)Use toxic then attack or use fly as well to dodge his ghost type moves if your using ghost type pokemons.

But What pokemon should I use? (thanks BTW)

any Pokemon will do but I think golurk or golett will be better for mysterious trainer.

brys18 pretty much answered all your question well,I will fill up what he missed.

To battle & gain most xp,you will need to hold off your opponent as long as you can.For that,nothing can replace ghost types pokemons.Golett/Golurks are the best to battle with.

1.Teach toxic to the pokemon which you will be using to help train your/others pokemon.For example,if you are willing to train your Articuno,teach toxic to the Golurk.Articuno have more typed attack damages,so I will suggest not using your Articuno at all for training purpose.
2.To learn fly you can either pay/ask other trainers.Or if you want to go for yourself,you will need a team of golurks(4-6) possibly at same level or atleast within 1k level difference of your party highest.
4.For most xp,definitely mysterious trainer.The higher level you opponent is,the more xp you gain.Mysterious trainer's pokemons are 10 times leveled of your party highest.
5.To beat Mew,you will need toxic,fly & MEGAHORN.Use toxic to toxicate Mew,then either use MEGAHORN(to quickly damage & finish the battle) or FLY(to play safe by not getting all your pokemon faint).

Question, who do you teach fly 2 and how will Articuno get exp. Which pokemon will need toxic fly and megahorn?

It's better to teach the rest of your team the move fly and megahorn
and Articuno can get the exp by letting him to the finishing/last move. Example, if you think that mew will faint for 1 last turn(because of poison from TOXIC). Sub/Change your Pokemon to Articuno. Then attack mew. If mew fainted. Articuno will get the exp.

Have those two moves on all of your training team,works better as you can use any move with any pokemon then.