I'm assuming you know how to record and play demos already. If not, that's not a problem, you can find a good guide right here.
Demo editing will let you do (almost) anything your heart desires in the world of CoH, but it takes a little knowhow on how demos are laid out. For a better understanding on demo editing, I suggest taking a look at this guide.
The simplest way to get your shot, which I'd generally recommend for demo editing newbies, is to simply fight any mob and replace that model with Requiem. Of course the easiest way in this case is just to go fight Requiem and record a demo, but if that's not so much an option here's what you can do:
Let's say you picked a fight with a Galaxy Adjutant and recorded the demo (since you're replacing the mob with Requiem, it's best to fight something nictus-y. We don't want him pulling out a hatchet now, do we? 
). Find the demo in your "client_demos" folder, which is in your "City of Heroes" folder.
Here's what the entry would look like in the demo (all of the numbers in this entry will probably be different in your demo, this is just for reference):
<font class="small">Code:[/color]<hr /><pre>0 2889 NEW "Galaxy Adjutant"
0 2889 NPC Council_Galaxy_Lieutenant
0 2889 POS -1119.5 0.015625 -5956</pre><hr />
All you'll need to do now is replace the model of the Galaxy with that of Requiem (only change the part outlined in red):
<font class="small">Code:[/color]<hr /><pre>0 2889 NEW "Galaxy Adjutant"
0 2889 NPC Council_Requiem
0 2889 POS -1119.5 0.015625 -5956</pre><hr />
And voila- your galaxy is now an AV 
.
Beautiful!! Thank you! And no, this'll be my first demo. But I figured I could get the rest of the instructions from guides
--Virtue--
My 50s:
Tigra Swipe (BS/SR/Dark Scrapper)
Galena Storm (Emp/Ele/Ele Defender)
Master Tolarin (Psi/Fire/Force Blaster)
And as for having the SGMate just fight him... Requiem needs to be knockbackable, and it needs to be on a specific map, and we don't want to go through the entire looooong arc again to get to the right map and right enemy who isn't KBable anyway.
--Virtue--
My 50s:
Tigra Swipe (BS/SR/Dark Scrapper)
Galena Storm (Emp/Ele/Ele Defender)
Master Tolarin (Psi/Fire/Force Blaster)
Got it! Thanks so much y'all!!
--Virtue--
My 50s:
Tigra Swipe (BS/SR/Dark Scrapper)
Galena Storm (Emp/Ele/Ele Defender)
Master Tolarin (Psi/Fire/Force Blaster)
I want to try and get a specific screenshot of one of my SG's heros fighting against Requiem. Would it be possible to set this up in the demo editor? Can you control named NPCs in the demo editor? I've never tried it before...
Thanks!!
--Virtue--
My 50s:
Tigra Swipe (BS/SR/Dark Scrapper)
Galena Storm (Emp/Ele/Ele Defender)
Master Tolarin (Psi/Fire/Force Blaster)