In order to guarantee Assist calls the same move every time, your entire team (other than the Assist user) has to be carefully constructed to use nothing but moves on the Assist exclusion list, with the exception of the one move you want it to hit.
The Assist exclusion list consists of:
Counter
Mirror Coat
Protect
Detect
Endure
Destiny Bond
Follow Me
Rage Powder
Snatch
Helping Hand
Thief
Covet
Trick
Switcheroo
Feint
Focus Punch
Transform
Bestow
Dragon Tail
Circle Throw
Roar
Whirlwind
Spiky Shield
King's Shield
Mat Block
Fly
Sky Drop
Dig
Dive
Bounce
Shadow Force
Phantom Force
Spotlight
Shell Trap
Beak Blast
Baneful Bunker
Metronome
Sleep Talk
Assist
Copycat
Me First
Mirror Move
Nature Power
Chatter
Struggle
Sketch
Mimic
Belch
Celebrate
Hold Hands
All signature Z-moves (like Catastropika) are also on the list, but the common ones (like Gigavolt Havoc) aren't, though they can't legally be in anyone's moveset anyway and would only be good for 1 power if they could. As you can see, building a team entirely with these moves is extremely limiting, leaving you with very little in the way of options in exchange for having that guarantee to effectively splice one move onto an unnatural holder who happens to learn Assist. Such teams almost always include a Ditto simply because its only move (Transform) is on the list, and transforming into your opponent is another way to gain access to powerful moves (and possibly setup boosts) that doesn't interfere with your Assist plan.
Prior to X and Y, all the moves between Dragon Tail and Baneful Bunker weren't on the list. This led to so-called "Fly-Cat" or "Dive-Cat" teams, where they would use a Prankster Liepard holding a Lagging Tail while the team was rigged so that the only move it could call was Fly or Dive. Assist is a status move, so barring priority moves, you would go first when moving into the invulnerable state, but Prankster doesn't apply to the second turn of charge moves (not even if they were originally called with Assist) so Liepard would move last when coming out, and hopefully the opponent would never get a chance to move while you were actually vulnerable. Fly had the advantage of more power and no immunities (not even Shedinja), but Dive had the advantage of fewer moves that could hit through your semi-invulnerable state (the big one was Surf, which most water-types had given up in favor of Scald in the same games Liepard came into existence).
Less common, but still occasionally seen, were "Whirlwind-Cat" teams where you would have Assist turn into Whirlwind with positive priority and blow your opponent away before they could ever move, but in order to get any productivity out of this you needed them to take hazard damage, and you couldn't afford to have any hazard moves on your team (Stealth Rock or Spikes) because they would spoil your own Assist. The solution was to have your Ditto transform into the opponent's suspected hazard setter and set them up after that, but if the opponent turned out not to have any hazard setters, the Whirlwind-Cat team became almost completely harmless.
Because of how disruptive those team styles could be to the unprepared, those moves in the middle were added to the list, so now both the Fly-Cat and Whirlwind-Cat teams fail to function at all. You'll have to come up with a different move to take advantage of, which you seem to have chosen Spore and that's fine. But Spore might not make up for the limitations you have to put on the other five team members.
