First off I just ask the obvious.
Have you played Gw1 before?, if you have and unlocked it in that game just transfer it over and it will be available from the get go.
Now if you only have GW2 it's still entirely possible to get. I was able to get Chomp Cannon through 15k packs, however in this case you may want to focus on buying Zombie only packs. preferably the 35k one on the end for the best chance of getting it.
It's just RNG, you will get it eventually.
Best of luck.
35K packs and perhaps a LOT of them depending on your luck with the RNG. I had all the legendary characters before I finished getting all the alternate abilities.
