Chances of doing anything lasting (i.e., above a Shaken/Stunned result): ~12.7% chance. It will take an average of 8 Lances to achieve a result that isn't ignored by Fortitude.
Note: this might be a little low as your opponent may not want to risk a Weapon Destroyed/Immobilized result morphing into a Destroyed. Then again, with only a 33% chance that'll happen, he may risk it.
Chances of destroying the blasted thing: 2.1%. Yep. Just under 50 Lance shots required to, on odds, kill one of these things.
They might make a mess of your lines, but if you're shooting to kill, you might just want to target the objective squatters next time.
This sounds simply like something you just weren't prepared for and got caught with. Those DS units won't come in until Turn 2, earliest. While Servo Skulls reduce scatter distance, they also disappear when you get within 6" of them - not exactly a problem for the DE.
It's also only a 44% chance that both of those squads arrive on Turn 2, even counting Psychic Communion, so far from an assured strategy.
Also, Night Shields.
