Always declare Machine-type with Array of Revealing Light so that Vellian's monsters cannot attack the turn they are summoned.
Since Vellian's monsters cannot attack immediately, Guardian Statue and Medusa Worm will allow you to control the board by removing your opponent's monsters from the field when they are flipped.
Remember to always use their effects to flip them back face-down for reuse next turn.
Only have 2 Guardian Statue/Medusa Worm on the field at most. Reserve one of your monster zones for Gravekeeper's Vassal.
Sometimes Vellian will have more monsters than you can remove with your Guardian Statues and Medusa Worms, in this situation use Acidic Downpour or System Down to remove all Machine-type monsters from the field.
Before your final turn, make sure you either normal summon Gravekeeper's Vassal or tribute summon Mystical Beast of Serket while Temple of the Kings is on your field.
Send Mystical Beast of Serket and Temple of the Kings to the graveyard to special summon Blue-Eyes Ultimate Dragon
Remember to use Anti-Magic Arrows during your final battle phase to avoid Vellian's Zero Gravity.
Immediately Summon Gate Guardian by tributing the monsters you get from Elements Unite. Gete Guardian should be in DEF position first unless Crowler will attack you with his monster equipping Ancient Gear Tank.
The next thing you need now is to activate at least 1 healing card so that Crowler does not burn you to death with Ancient Gear Tank. Once you regain enough LP, it's better to switch the position of Gate Guardian to ATK.
Lastly, you need to equip Gate Guardian with at least 2 Horn of Unicorn so that Crowler cant overpower your monster, both in attack and defense position.
After that its now only a matter of getting to zero cards quicker. Use Shard of Greed for extra draw power. The combo of Gravekeeper's Recruiter and Wonder Wand to get 3 cards out of your deck. It is best to search out another Gravekeeper's Recruiter if possible to reuse the combo.
Remember to use Anti-Magic Arrows at the start of your final battle phase.
If you can just be more honest and tell us it's YOU who's having a problem unlocking crowler, then I would have gladly helped you. Since you're so bent with preserving your image and reputation which doesn't even matter in this site where there's anonymity, I have to pass up on your question so you can learn one bit.
Since this "community" is so bent with critisizing others, I wouldn't admit anything to be honest.
To sorrel, I don't know which are the missions for unlocking him. But it's usually just playing around. For comeback victory, just use a pvp or beater deck, get some damage and then win
I modified turtle farm, to use against crowler. With some slight modifications, works well against joey, pegasus and ishizu. Just need to get spirit barrier and duck dummy in and, plus any card to help duck dummy have its gemini
If you have Gravekeeper's Vassal, its attack is considered effect damage. Just boost its atk and use it to deal 2000+ damage. You could also use Flame Wingman to destroy a 2000+ atk monster. I'm sure there are more, but those two are what I can think on top of my head.