Thus far I have had a platoon sergeant take down Ku'gath the Plaguefather, and a Blood Angels Librarian with a Forceweapon take down a Farseer using his forceweapon on 3d6. So those were some very cinematic battles that were fantastic challenges that ended up defining both of those games.
Imperial Guard vs Tau w/ Daemons
In the first of the challenges, it was the re-rolls that took down Ku'gath. With 4's to hit and 6's to wound, I was able to deliver the final two wounds to the Daemon with a single guardsman, and might of pulled one the greatest melee combats for a simple platoon sergeant. The Daemons throwing themselves at the Imperial Lines were destroyed with the Plaguefathers demise, and the remaining Tau forces were thrown into a defensive battle to attempt to hold onto their primary objective. Attack runs from Vendetta Gunships and massed Imperial firepower were able to remove the final vestiges of Tau guarding their objectives.
Imperial Guard w/Blood Angels vs Dark Eldar w/Eldar
The second memorable challenge was a large platoon with a blood angels librarian trying desperately to kill a farseer, and thus end the psychic stranglehold the Eldar Farseer was holding over the Imperial Lines. Already wounded, the Librarian held only a small advantage over the Unwounded Farseer on a jetbike. The Farseers psychic might allowing it to re-roll failed saves, was making the battle desperate for both sides...... Adrubael Vect was approaching with a squad of Incubi, and if the battle did not end soon, Vect could easily change the flow of the battle.
Of course with only the Farseer left of his seer council, the Librarian had moral support, and was able to get three hits into the farseer with two wounds. Amazingly one got through to the Farseer after double failures on its invul save, and the librarian seeing his chance for victory attempted to activate his force weapon. Failure on 3 dice meant death for the librarian, while success meant the battle could be won. The force weapon was successfully activated, and the Farseer was struck down. The resulting round of battle unleashed powerful psychic energies into the enemy, and Asdrubael Vect and his incubi were broken and ran down by Blood Angel Assault Marines. The battle turned, and the Imperial Forces were victorious.