You gotta have optimized weapons and armor and max endurance and smart pulling. Armor stance is a must and armor aura can be helpful. Also in addition to healing Gamma, First Aid is extremely useful basically turns your Stamina into a second mana pool for healing. Bandages are super cheap and spammable for healing and also give a Health Regen long term buff. Keep that health up and farm all day. I got to 55 but I respecced into rifles just because ranged weapons are convenient. In parties though, melee is really effective because party members can spam heals on you. Benevolence heals double if you cast it on someone vs self cast. Makes it more Gamma efficient. We can level together on the Reboot if you're interested I'm going rifles though for convenience, but I've soloed the elite mobs melee while leveling. You can still shoot guns in melee and range just opens up your options. But I will always appreciate melee combat
You gotta know the weapon recipes, some weapons that require level 15 will have less dps than a rare recipe that only requires level 12