Yes it's possible to RNG stationary Pokemon. In fact, that's the easiest thing to RNG and is what I recommend doing first if you're still learning to RNG-abuse. I say it's easy because it takes fewer steps than other RNG-abusing.
RNGing in the Safari Zone is possible, but to RNG wild encounter pokemon, you also have to take into account things called Encounter Slots, which is something only really controllable through the use of Sweet Scent. Here's a guide that includes Sweet Scent use and explanations of Encounter Slots: http://www.smogon.com/forums/showthread.php?p=2830548
I would say, however, that the easiest way to get your flawless Cacturne is to breed for it. Encountering a wild Pokemon with flawless IVs is very hard - much harder than just breeding with a flawless Ditto and having the egg inherit IVs from it. RNG breeding is still complex, so if you can practice by RNGing stationary legendaries, I'd recommend doing that a few times.
I understood the seeds/frames and everything else. Also, is it possible to RNG a pokemon in the Safari Zone in HGSS? I really need a perfect Cacturne, but it's impossible to RNG it in the sandstorm(DPPt)
Also, is it possible to RNG stationary NPC pokemon. Like such as...Mewtwo/Heatran/Legendary Birds/ and also Rotom?
I want to see if I can get a rotom of the right nature.
In order to RNG legendaries, I would have to use the re-battle code. I don't own an AR though, so unfortunately my RNG services are restricted to Pokemon that I can either breed or catch.
I can breed a Rotom and give it W-form, but once it goes into the wifi-room, it reverts back to standard form, so unless you have Platinum or HeartGold/SoulSilver, there's no way for you to get the different forms.
And I would be glad to help you learn to RNG. But since it's so complex, it would be very difficult to help you over messages. My recommendation is to study these pages first: http://www.smogon.com/ingame/rng/dpp_rng_intro
Then I can help you if you have specific questions. Or I can even meet you on an IRC channel sometime to walk you through it. The hardest part is really just understanding the concepts like "seed," "delay," "frame" etcetera. Actually doing the RNG-abuse is pretty easy after that.
Hey, is it possible to request a legendary with a specific nature? I need an Impish Nature Uxie.
I would also like to request a Rotom-W[Timid] because I don't have platinum version, and I don't have access to them without it. I stiil have TMs in stock if you need it, and I can EV train your pokemon in return. I have poke'rus and choice items if you need it.
I'm merely asking you this because it's impossible to retain these pokemon in the game normally, and I already caught Uxie.
And also, can you show me how you RNG'd that Timid Roselia with those IVs? I really want to learn how to RNG my own pokemon.