I can grind rebirth PRETTY quickly, my fastest rebirth being around an hour and a half (10:30 AM - 12:00 PM~). So when my friend asked how I grind so damn fast, I figured I'd make this for everyone :D
REQUIREMENTS: Max Enchant D4C/TD4C, or KQOH/TKQ if you don't wanna use those two, TULF or Kars Hamon for Spec, and a friend with TD4C/D4CR. Zamus or Time Dispersal is recommended by me. 2x EXP is also HEAVILY recommended, and it was used in my fastest rebirth.
Before you even rebirth, go to the final quest in Death Island. Get the quest down to 1 kill (or just use reb after you get quest if you're worried about it despawning). Use rebirth arrow once you do that. You'd then use Full Love Train/True Love Train, and have your friend do the same at the Quest. You're friend would lower the NPC(s) and then you'd kill them. Do this until you finish the quest.
If you have 2x EXP, you'll be put in a level where you can do Morioh quests. Do them normally, and I'd switch to T/KQOH to fight Kira. Criminal Masters is where Zamus/Time Dispersal will come in handy. You can use Fast Slice/Time Cut to collect all of the enemies and then use a bomb or clones. Do this until level 400, which then you'd enter Italy. What I normally do is grind Gangsters + Diavolo until level 625, and then do the Prisoners Quest in Prison. The Newbie Prisoner Quest is quite weird, and has a back placement to grind at. Grind all quests in Prison until level 725.
Death Island, level 725. Grind Magma Guards until Magma Monsters, and Magma Monsters until Beastmasters (850 Quest). Do level 850 quest and target the 5 NPCs in a square. On Magma Monsters, Fast Slice is great for gathering NPCs into clones/bomb(s).
Reasons against other things:
D4CR Clone damage is low as hell, so don't bother with it. It also doesn't have a move that can collect NPCS like T/D4C, KQOH/TKQ do. This is the same with GEOH, it's Ult Jungle will not one shot an NPC unless max enchant, dio skin, and a class cannon build.
While Light Blade can be switched with Time Dispersal/Zamus, but it takes much longer than Fast Slice would.