Round ONE.

Abaddon charges Carnifexes.
Abaddon rolls a 2 for daemon weapon and so has 7 attacks.
He hits on 5 attacks, wounds on 5.. one Carnifex dead, one with three wounds remaining.
Two fexes strike back, four attacks each re-rolling missed hits to hit 6 times wounding 5 times.
Abaddon saves all 5 with his 4++ !
O. M. G.
Abaddon hits back rolls a 3 for 7 total attacks now.
Wounds on 6, another Carnifex dead, last one standing with one wound left..
The angry fex hits back, hits 3 times.. wounds three times,
Abaddon fails two 4++ and takes two wounds with two left.
Abaddon rolls a 4 for his daemon weapon and hits back with 8 attacks.
Hits on 6, wounds the Carnifex 5 times..
A B A D D O N V I C T O R I O U S !

Round TWO.
Carnifexes charge Abaddon.
Abaddon rolls a 6 for daemon weapon for 10 attacks.
Hits on 7, wounds on 5, one Carnifex dead, one with one wound.
The remaining two Carnifexes hit back with 10 re-rollable attacks, hit 9 times, wounding on 8.
Abaddon fails three saves, leaving him with wound.
Abaddon rolls a 6 for daemon weapon for 10 attacks.
(Another O.M.G. moment!)
Hits 9 times and wounds 9 times = two dead Carnifexes.
Tyrannids everywhere are crying.
A B A D D O N V I C T O R I O U S !

Round THREE
Starting toe to toe.
Abaddon rolls a 2 for the daemon weapon = 6 hits and deals 6 wounds.
One carnifex dead, one with two wounds remaining.
Two carnifexes hit back with 10 attacks and fail horribily only hitting 5 times even with their rerolls.
They wound 4 times, but again.. Abaddon fails 3 saves leaving him with one wound.
Abbadon rolls a 5 for daemon weapon for 9 attacks.
Hits on 6, wounds 6 times = two dead carnifexes.
A B A D D O N V I C T O R I O U S !

In a pre-game interview I asked my friend that plays Nids what he thought would happen and he immediately said Abaddon would wipe them out.
Seems as if he was right...