it is hard, or impossible, to find spawns with only one maturity level nowadays.
Anyhow. S and SE East Scylla Mountains you can find atrax young. There are also some big snabelsnots and some cornundacaudas though.
Far ENE Echidna I think the most pure atrax spawn is located. Mostly youngs but even some mature. On the way there you will encounter some berycled, feffoids, hiruyus and exarosaurs. It is also possible to reach that area from Caravan and then it might be possible to sneak around the mobs on the way there (snabelsnots, snarksnots, cornundacaudas and allophyls.)
I dont have the coords but it is in the gap of the mountain range ENE Echidna.
I think rascal with 2a plates would do. When it comes to suitable melee weapon, well, when it comes to melee I am way too nOOb to have any suggestions.
[Edit: Yah, you beat me in speed Fiona
. And, how could I forgot the fine hunting ground on TI?!]