Enemies don't really get strong, they just get diffrent.
Oh no, How Dare a game challenge you to learn how to play. You should be getting so strong that NPC's are like Shooting fish in a barrel.... *sigh*
You need to learn how to fight things. If by rocket guys you keep referring to, you mean the big guys with the backpack as their weakness, then they are pretty easy. 2-3 sniper shots in to their backpack, they kneel down opening their mask, shot to face, easy 7-11k damage for the kill. This can even be done while fighting 3 or more of them, you just have to use your head and use cover...
Just remember, Cover is your friend. This is not CoD where you can run and gun 20+ guys and live cause they all seem to not be able to hit the broadside of a barn.
once you have maxed out the xp on a weaopn your weapon skill stops going up. you need to replace it. this becomes less of a hardship as you get higher ego as athe weapons tend to come with more open mod slots saving you on ark bits which are better spent on lock boxes.
they get harder because until you face the 99ers and dark matter the game is honestly rather easy.
On the topic of removing mods, the weapon must not be currently equipped or that option will not be there in the salvage matrix screen.
It costs Ark Salvage to remove the currently installed mod and takes a few minutes, but after you have removed the mod, it will be destroyed.