The Shaman's tactic is fine.
Most of the shaman powers are ranged or have the spirit keyword.
This means the shaman can stay away from the front lines and stand in a slightly safer position, where the only way to harm him is using ranged attacks or bypassing the defender.
It's actually a good thing for the group if the shaman stays back and tries to get less damage as possible, since he's still able to contribute from the distance. He needs to stay alive so that he can heal his comrades.
His panther is also good in melee, because attacking it is a really bad choice, especially if the paladin is using Divine Sanction.
Single target attacks either come from faraway enemies, who'd better just attack the shaman or a striker, or come from melee strikes and the paladin (especially if using Divine Sanction) where the paladin deals them almost as much damage as the Shaman will take if the panther goes down.
This is because, to kill the panther, they need to bypass a treshold or their attack is wasted and, even when the damage is enough, any extra damage is wasted and the Shaman gets 5 less damage than that anyway.
Area/close attacks, the only ones able to bypass Divine Sanction by hitting the paladin as well, on the other hand, just do nothing to the spirit.
Removing the spirit from combat only means the Shaman has to use a minor action to re-summon it, and since the Shaman doesn't really need to move from its same spot, he usually doens't ever lose his healing capabilities.
What could be problematic is if nobody else in the party is melee but the paladin, but this is a party composition problem that has not much to do with the shaman tactics alone.
Our party has a chaladin, a bear shaman, a slayer and two other ranged strikers (a cosmic sorc and a feylock) and it's working pretty fine. As a DM I rarely had the impulse to attack the spirit companion, because it's often useless to do so (that is, unless it's blocking some important path on the battlefield and I need it clear for a turn).