Take an Earth Titan and reskin it as a "huge headless stone construct." Its body is covered with aberrant mystical runes, carved with impossibly fine tools - an expert eye can tell you that the entire construct was carved from a single gigantic chunk of rock with disintegration beams.
Each construct is a level 16 elite brute. Pair it with a lower-level elite beholder, a level 13 Eye of Flame. The Eye of Flame rides around as the "head" of the construct, protected by a "helmet" of the same magical rock, allowing it to stick out its eyestalks and use its central eye, but giving it considerable protection. Say the Eye gains +5 to all defenses as long as the construct is alive.
The boulders the construct hurls are loaded into grooves along one arm; big enough to threaten a man-sized target, small enough that the construct can have plenty of them in the grooves. The boulders are actually stone spheres with eyes carved into them, like nascent beholders, and they burst into flame when they hit someone. Replace the "daze" effect with an additional 10 fire damage added to the regular impact. Combined with the Eye's "no hit roll required" ability to inflict flammability on things, this becomes more potent.
Make the Eye of Flame always go 10 before the Construct (they use the Eye's intiative roll to determine these numbers), allowing players to note things like "the Eye just made our Wizard flammable, so we need to keep the construct from throwing that fireball at him."
Give the combination several vulnerabilities...
1. If the Eye is dazed or stunned, so is the Construct it's piloting.
2. If the Construct has no Eye, it stands idle and helpless.
On the other hand, if there's more than one of these combos present and one loses its Construct while the other loses its Eye, the remaining Eye should totally be able to dart over and start piloting the intact Construct.
As a likely stunt, allow PCs to take over abandoned Constructs too, using telekinetic power, Thievery checks, or other tricks.
And, hmm, if a PC wants to jump up on the Construct's shoulder for an easier shot at the Eye, make that bypass the Eye's +5 defenses but at the cost of granting combat advantage to the Eye and the Construct.
Depending on your party's level and numbers, throw 2-3 of these combos at them at once and watch the fun.