Trapped and Outmaneuvered
A cornered enemy will fight back even against overwhelming force, if their only chance of escape is through said overwhelming force.
Depending on the tactical savvy of the commanders on each side, the rebels could somehow lure the conscripts into an extremely poor situation, trapping the army between the proverbial rock and a hard place. So even though the conscripts want to break and run immediately, they can't.
As an example, it sounds like the conscripted army is on the offensive against a rebel-held fort. The rebels could initially under-play their strength, allowing the conscripts' commanders to exhort them into assaulting the walls. Once the conscript army is committed (siege towers and ladders in place, battering ram smacking the doors, etc.), the rebels unleash their full capability. Archers come out of hiding and begin firing, boiling oil, flame traps, etc. The conscripts immediately realize that they've been had, but they can only disengage from the siege so quickly.
Then the reserve force the rebels had hidden nearby takes that as their cue to charge. The rebel flanking force hits the conscripts' back ranks, scattering or overwhelming the command structure and shattering any chance of an orderly withdrawal. By the time the conscripts are able to escape the encirclement, they could easily lose a quarter of their number or more.
And since you want specific over-powered rebels to be killed in the battle, they could be the ones leading the flanking charge. It's the highest-risk position on the rebel side, and if they're that powerful they might see it as their duty.
Other possible things that could prevent the army's escaping:
- fire arrows ignite a forest fire behind them
- a critical bridge is destroyed, trapping them against a river
- alternately, a triggered surge of said river carries away the boats they'd used to cross it.
- in mountainous terrain, the rebels could use siege weaponry or pre-set traps to trigger a rockslide.
etc., etc., etc...
Something similar (minus the fort) was used historically by William Wallace against the English in the Battle of Stirling Bridge. The Scots hung back while the English army began crossing a narrow bridge, then rushed them once a few thousand troops had crossed over. The portion that crossed was trapped against the river and annihilated, and the remaining English army on the other side of the river destroyed the bridge and withdrew.
Or in fiction, take the Battle of Helms Deep from Lord of the Rings, except give the orcs just enough time to commit to hitting the walls before the Rohirrim and Gandalf charge their flank.