Two leman russes at 500 points is pretty rough. There are not enough points for your opponent to bring enough to likely because to deal with them without list tailoring. You should add more infantry.
Don't do Pask as an Executioner. And a Battle Tank is a bad buddy tank. The battle tank and the demolisher make a better solo tanks because of their price, blast template and the ordinance rule. The Buddy tank should be a cheaper tank or an Executioner. The reason being Pask's unit is a bullet magnet so you need a cheaper tank to take the hits because he is probably going to die quick. The Executioner is a decent choice as a buddy only because preferred enemy will protect it from gets hot. Pask himself is best in a Punisher or a Vanquisher. Because it makes the best use of his rules. In these variants, he is an amazing tank and infantry hunter. So the best combo for Pask is Punisher, Melta, and Lascannon with camo. An Executioner as a buddy should probably be naked because he will probably die and you really don't need extra shots when you are slinging three plasma, though having side cannons isn't bad. I personally like the Eradicator or Vanquisher as the buddy, but it is kind of personal preference.
Manticores do not need a dozer blade. They really should be in the back corner never really moving getting off its missiles as quick as possible. Camo nets are kind of a rough too because you should be hiding outside of LOS and if they do get your in sight, you are probably dead anyway. With an Aegis line a camo net isn't a bad choice.
Don't squadron your Vendettas. There is no benefit to unless you are hurting for FOC slots which you are not.
I would go ahead and add that third melta in your veterans. I also find ML/flamers to be more useful than bolters on chimeras. The reason being you can't fire two weapons at full BS so one weapon is going to be wasted. The flamer can be very decisive if used in the right time by ignoring cover and auto hitting. The Multi-laser with Str 6 can sometimes kill T3 characters in one shot or wound vehicles that the HB can't touch. Most cases the ML will do just as well as the bolter.
If you can drop some points maybe fit a Wyvern in do so.