You have a dual-leader party, which is (in general) a really good idea. The cleric, the ardent, and maybe the hexblade need to realize that there's no-one stopping them from drawing aggro. They are also the theoretically hardiest members of your group. Your ranger may be, if it's a two-weapon strength based longtooth, but I'll assume archer for now.
The simplest thing is to make your ardent into a not completely worthless defender.
She should consider:
- Energizing Strike (not hugely necessary in a dual-leader party, but always useful)
- Intent Laid Bare (a pseudo-defender power, making the ardent quite sticky. The ardent, of course, needs to be using really good armor and a shield, but that has to be the case in any event.)
Wave of Fatigue: another pseudo-defender sticky, I'd say that intent laid bare is better at being a defender, while wave of fatigue is better at complementing a defender.
Guarded Prison: This gives you a punishing pseudo-mark and is required for the ardent to be able to be a "defender" Realize, of course, that most of her healing will be self focused.
A strength based cleric, or a warpriest are also acceptable off-defenders. Leaders naturally attract pain, and a warpriest can survive long enough to give a credible imitation of defender.
Your mage and psion will actually be in rather dangerous circumstances, as without joint planning they tend to stomp all over each other's specialties. However, if both of them focus on forced movement rather than damage (usually a good idea for controllers, since damage is a resource-trap that strikers already are better at) they can credibly force monsters around the two "defenders" (leaders) such that the strikers can focus fire while the leaders heal themselves.
All meleers in the party should also consider taking the "Primal Guardian" theme, as that gives them a mark with a little punishment attached. The ranged characters should consider templar or veiled alliance (but mixing and matching is probably bad from an RP perspective) (except the psion who simply should take noble adept)
My recommendation would be for one or both of your strikers to reroll, but I'm quite strongly biased against strikers in the first place (I view them as the least necessary role, though it is nice to have 1 around...) If not, everyone should realize that they are far more responsible for their own defense than would otherwise be the case and focus their resources accordingly. The psion needs to focus on attack debuffing, the ardent on pseudo-defender powers, the mage on movement debuffs, and the strikers on defense.