"If Deoxys get's close he gets shredded by the stardust dragon and a point black last prim"
Defence form, sorted. Stardust Dragon becomes Rayquaza 2.0
"that the terrarian could easily take back"
Not if dups picks them up first.
"high-level danmaku"
High amounts of Dups more then the Terrarian can produce projectiles.
"
because they a lot of terrarian's projectiles have selective intangibility and would just phase through those barriers" Never seen this before, I've played through the entire of Terrarian, even Hardcore, never seen this, ever.
"The terrarian is a genius and has already fought enemies with weak points even less obvious"
Actually, I've yet seen anything that makes the Terrarian a "genius" beside his ability to use weapons and craft them. And Deoxys is a Super Genius, being a psychic type, being able to control at least thousands of Duplicates and being able to outwit the likes of Rayquaza
And still, haven't spoken about Mirror Coat reflecting.
"the Terrarian homing attacks have way higher danmaku and swift is deoxys weakest move"
Actually, one Swift isn't the weakest move, by far, and swift can go on as long as the person needs, so at least matching Danmaku and it's coming from a being who one shotted Rayquaza to the floor and forced him to rest for a minute before getting back up, not only that but Deoxys has the highest Attack and Spe-Attack bar Mewtwo's mega evolution forms.
"and completely incapacitate Deoxys for a small amount of time, but that would be enough for the terrarian to land whole danmaku of homing attacks or teleport and do a point black last prism/nebula arcanum/celebration"
Presuming that it didn't just hit the Terrarian, and if you say the Necklace (Increases immunity frames) then that would mean that the Terrarian has either a weaker Stardust Dragon and weaker magic or that he doesn't have the necklace and took A LOT of damage.
"if you mean the move snatch, that works only on healing/stat boosting moves, if you mean literally Deoxys snatching the last prism, he wouldn't be able to use it has he doesn't have mana"
All people start off with 20 mana, enough for at least 1 second of use against the Terrarian and then he can give it to a Duplicate to run of with.
I'm also giving screenshots from the movie, showing different examples of Durability, attack, genius etc.
http://i.imgur.com/jE4EY0r.png
http://i.imgur.com/iBFLpPa.png
http://i.imgur.com/EbPGd95.png
http://i.imgur.com/QvpEwD3.png
http://i.imgur.com/ScQKa67.png
http://i.imgur.com/nWf8NGw.png