Can these mods only be obtained by using Nekros skill, or can they be obtained without using that skill with low drop rate?
The Reflex Guard mod is obtained as a rare drop from Corrupted MOAs. They can also be found as rewards from Tier 1 Void Survival. The Berserker mod is obtained as a rare drop from Corrupted Ancient Healers, or can gotten through transmutation.
You can use Nekro's Desecrate skill on these two types of enemies once they die, in order to have a second chance at receiving one of these mods. Personally, I suggest you do void survival, defense, or exterminate missions for the best shot at getting these mods.