There's usually 2 ways to approach CB.  The first is with an unkillable team and the second is with a sustain team.  The latter apperas to be what you will need to aim for, since I assume you don't have any Maneaters/Demythas/Towers/Warcasters.

With sustain, a lot of this will come down to gear, as from what I can see you already have the champions that should be able to 4 key UMN, and might even be able to 3 key it.  There's a couple of things you will want to check off as part of the team build

• You will want decrease attack on CB all the time.  If this ever wears off then you will take significant damage.  
• You will want increase defense on your team all the time.  Your team will ideally need to be sitting around 4000 defense with increase defense on, so about 2800 defense without.  If you have someone like Brago in the team then you can reduce this further because of his passive (probably about 2600 defense)
• You will want ally protect.  This skill massively mitigates damage, and therefore keeps the team alive longer
• You will need to think about how you plan on dealing with the stun which comes around every 3 rounds.  Block debuffs is usually the best way to deal with this, or alternatively you need someone to simply wear it.  Stun will not only make someone miss their turn, but will also not refresh the skills for that turn.  This means that if someone like Iron Brago gets stunned, then his 3 turn CB on increase defense will take 4 turns to fully refresh, which means 1 turn of attacks without increase defense.
• You will need some sort of healing capabilities.  This can include leech on CB, lifesteal gear on the champions, or simply a champion that heals people up
• Each of the champions needs to be level 60, with either Warmaster or Giant Slayer masteries, depending on whether they are single hitters or multi.
• Other debuffs like decrease defense and weaken are good to improve damage output, as is other buffs like increase attack, but these should only be considered if you can lock in decrease attack and increase defense.
• Finally, you will want to consider the speeds of the champions.  You might not necessarily want some champions to be going twice as fast as others, because otherwise buffs will drop off and things will go out of order.  

Key standouts to me are:

• Toragi - Ally protect, poisons and decrease attack on his A1.  Turn off his A2
• Brago - When booked, can keep increase defense up 100% of the time on your team.  Also brings decrease attack.  His passive also improves defense to improve survivability.
• Brogni - High shields for survivability plus block debuffs.   Just make sure that he uses his A3 at the right time so that block debuffs is on when CB does his stun attack.  Brogni can also provide healing through his shield, though I wouldn't rely on it.  At a 1:1 speed time (i.e. all champions at about 190 speed), the reflection of damage from Brogni over 50 rounds comes to about 6 million damage, so not enough to get to 3 key UMN,
• Ninja and Geo - High damage source.  If you do plan on bringing both you will need to watch the sequence of HP burns.  You need Geo's HP burn to go out before the A1 of CB, and still be on for A2, so that you can get the full damage reflection output from both AOE's.  Ninja can also be quite hard to speed tune due to his A1.  Also, if you bring Brogni in with Geo you need to make sure he doesn't override Geo's HP Burn either with his A1.
• Poison is a good source of damage, but in my testing at a 1:1 speed tune with CB (i.e. all your champions speeds are around 190) it can usually only bring about 10 million damage over 50 rounds (this was with Frozen Banshee, and included some counter attacks).  A nice chunk, but not enough by itself to get to the 24 million needed for a 3 key.   You need an additional source of damage, such as Geo, Brogni or even raw damage output.

I won't repeat what Rose said - good advice there. My two cents:

Brogni (block stun)

Brago (def up, atk down)

Rhazin (def down, weaken)

Toragi (ally protect, poison)

Ninja (damage)

Only part that gives me pause is that you have no Leech. But I think that Toragi and Brogni together can provide enough healing.

All great advice above.

Although I am unsure that Brogni's heal is going to be anywhere near enough, and Toragi's heal is only to himself, so likely you will be bringing lifesteal gear on some to all of your champs for a while, unless you end up using Ghrush for the leech (and everything else he brings :) ).

I would probably start with Brago, Brogni, Toragi, Ninja and Rhazin too.... but consider getting Ghrush in there for the leech. If you have Uugo stashed away, she makes a great combo Dec Defense, Cleanser, Inc Attack blocker and Leecher all in one. :)

Is Rhazin really deserving a spot unbooked, though?

I will be giving a spot to Tayrel for atk down and def down, Toragi for ally protect and passive poison and Ghrush for leech.

For DPS I am inbetween Ninja and Venomage.

That leaves last spot for either Brago or Brogni.

I have 2 lego books so neither Brao nor Brogni nor Rhazin can get booked.