Flyrants are very hard to kill. I'm both a Tyranid player and a CSM player and frankly, it's a bad matchup. Havocs with Autocannons are the general use option and about as good as it gets, but against a 3+ armor you're not really guaranteed anything even though you have some good chance to get hits. Flak Missiles are the same problem so don't bother with them. Nothing in the codex is designed to hurt a FMC of any kind really.
Consider a Renegade Knight, though Flyrants in groups can and will murder a Knight in about two Turns, but that Skyfire Cannon up top or even the S8 AP3 rockets without Skyfire and the Gatling Cannon at AP3 fire enough strong shots to really hurt a Flyrant. Keep in mind that a Jinking Flyrant is a worthless Flyrant. If you make two Flyrants Jink, you're ahead of the game even if you deal no damage.
The only Forge World Flyer I can recommend against Flyrants specifically is the Fire Raptor. The others are pretty weak and without a Contemptor Mortis or Deredeo, it's not like you have some good options. As a Flyrant player, I hate seeing Fire Raptors on the table because of the damage they can dish out.