I've been also wondering about it and actually I'm also not so sure about this ballista even though the guide is pretty clear. My conclusion is simple, but I'd like to get a confirmation somehow:
Requirements for this illusion ballista are: +7 ballista , 50 illusion stones and 100 shinig spores. Everything is clear to this point but the main thing is that the only ballista  you can get is from the socket enchant npc (Seiyablem) by creating a slot at 20% success rate...(kek)
So basically process is like this:
1. Get ballista from Dracula at (0.5% drop chance) - to compare, i was farming some 0.5% yesterday and I had to kill around 500 mobs to get it.
2. RNG for slot at Seiyablem NPC (20% chance success - if it breaks have fun getting ballista again)
3. If somehow you get ballista  refine it to +7 for illusion version (if it breaks have fun getting ballista and slotting it again hehe..)
4. Have fun upgrading it again to +7/+9/+15 OR repeat the whole process again if it breaks :^)
As I see the same goes to inflitrator  (but at least this one has higher chance to drop from non MvPs) and ancient cape  (which also drops from dracula 0.10 or dark lord 10% - way higher chance tho).
Could someone confirm if it works this way? Because if yes then thanks, nice update but I'd probably have to live in game to actually get it by own lel (at least this specific set, most of the other items seem pretty alright to get so kudos for that). Or maybe there is some other way that I'm not awared of ?___?. I'm seriously asking because I was joking about it yesterday with my friends but I literally see no other way.
Also is there any legit way to check at which point illusion aoa + illusion balista set got better than +15 bxb? Was doing some math yesterday but I gave up at the end, maybe someone is able to enlighten an idiot like me.