There are NO characters in the game that start with more than 10 Rage, so how are you supposed to play the 3 unbluffable cards that require more than 10 Rage? BOOSTING!
The 3 monster cards.
Cleft in Twain
Rage 15, damage 12
The character cleaving his opponent in twain must have a weapon. This Combat Action may not be bluffed.
(nothing can play this naturally)
Rage 13, damage 10
You tear some fool in half. Only usable in Crinos form. This card cannot be bluffed.
(Incarna Avatar and Zmei can play this naturally, but cannot be made your Allies)
Rage 12, damage 9
When all else fails... punch right through some chump's sternum and give his heart a wrenching. This card cannot be bluffed.
(Illganwasi baKalahari can play this naturally, but cannot be made your Ally)
The easiest way to get the necessary Rage for HB and RA is with Searching for Weakness. Play it during combat, get +2 Rage. Play both, have +4. It cycles through your deck and is usable by every single creature in the game. Play both in a combat and any creature with a natural Rage of 9 or more can throw RA, anything with an 8 or higher can thrown HB.
But we're shooting for the moon here, you want to play Cleft in Twain! The Rage isn't the problem with Cleft in Twain, its making sure you have that weapon!
The Black Spirals are actually pretty easy to do, even without Searching.
Zhyzyack or Longtooth Soulkiller are your best bets, give them a Blood Dagger to fulfill your weapon requirement. (grants +1 Rage as well)
Zhy starts at 10 Longtooth starts at 8. The vampires Count Vladimer Rustovich or Allonzo Montoya also work well since they can use Black Spiral Dancer gifts, but not the totem.
Blood Dagger +1 (equipment- almost every Wyrm character)
Beast of War +3 Rage (Totem, BSDs)
Horns of the Impaler +2 (gift BSDs or Fomori)
Dr. Mordicai's Home Chemistry Set +1 (equipment- almost every Wyrm character)
Totem Form +2 (gift- BSDs)
Experimental Cybernetics +2 (equipment (ANY wyrm character)
Include Prentice Turner in you pack with any of the your BSD choices and you'll start with +1 Rage. (remember Prentice gives himself one)
If you're so inclined to use him, you CAN actually get him to play Cleft! He starts with 3 Rage. To fulfill the weapon requirement, give him the Blood Dagger. That takes him to 4 to start, before you get crazy...
Assuming he's paling about with the BSDs Beast of War is out, that takes him to 7. Add Benefactor's Boon (Pentex, Cults Gift) for +3, taking him to 10. Give him some Experimental Cybernetics, +2 to 12. Use Ghouling ((Pentex, Cults Gift) for +1 (and he'll regen), 13.
He'd still need Searching for Weakness, but your "backup" could be throwing down Cleft in Twains after you lose Zhy or Soulkiller!
(Include Michael the Lost in your pack and start him with Experimental Cybernetics to switch to your heavy hitter)
Other possible Wyrm choices is Fara'un Shark who can use the Blood dagger, Chemistry set, Horns, and the Experimental Cybernetics. However, you can get them the last one by putting Prentice Turner in your pack or including a BSD character than can get the Beast of War.
Add Searching for Weakness and it becomes ludicrously easy to get Wyrm characters playing Cleft in Twain. They've got so much spare Rage, they ooze Rage!
Anyway, to Gaia. You need a weapon of some sort to play Cleft at all, so it might as well be the Grand Klaive for the +1 Rage. Gungnir or Dhul Fiqar are also viable choices, but limited to specific tribes.
The Get have the easiest time since with Gunnar, Golgol only needs 2 cards to hit Rage 15 (Gungnir/Grand Klaive and Gunnar). One more card and you've got a lot more in striking range.
Battle Song +2 Galliard
Gibbious Moon will give you +1 against creatures in Hunting Ground
Inspiration +1 Ahroun, Silver Fang
Might of Thor +2 Get of Fenris
Power of the Ways +1 Philodox
Gunnar Draggurbane +4 Get of Fenris
Leukippes +3 against males (Black Furies)
Caern of the Sentinel +2 when stepping in for Victims- Get of Fenris
Experimental Cybernetics +2 Glass Walkers
Gungnir +1 Get of Fenris
Fang Necklace of Fenris +2
Toga Party +2 (combine with Rite of Summoning)
Rite of Glory +1
Crusade +1 against specific creature type
Dhul Fiqar +1 for Silent Striders
Primal Anger +3 Rage Metis, Ajaba
Fatal Flaw +1 Lupus, Shadow Lords
King Albrecht is also a decent choice since it not only has 10 rage, he starts the Game with the Grand Klaive giving him 11. He just has trouble getting the rest of the way there... he either has to use Searching for Weakness or you need to build around boosting him to there. He looks easy, but really isn't.
The rest of the tribes are a little bit harder, but not impossible to do. The real key is that weapon, which if you don't have it, it doesn't matter how much Rage you have. Characters that start with Weapons thus are ahead of the game. King Al and Anna Kliminski are thus excellent options. Anna has an even harder slog than Albrecht, but she can get there.
If you ignore starting with a weapon, you have several Get characters that can get there without too much problem. By focusing on Get you could reasonably have 2 characters that can play Cleft in Twain!
Other tribes have a harder time, but can cobble something together. Thousand Cubs, Joseph Roars as Bull, Stalks Death, Blood on the Wind,and Julisha of the Thousand Masks could also get there, but would probably be the ONLY character in the pack that could play it, and it would take awhile. You're better off focusing elsewhere unless you REALLY want to be sneaky somehow. Heartbreaker and Rent Assunder can reasonably be played by these guys and is quite doable, but Cleft is really only playable if you want to PROVE they can play it.