EZist way to get this is Act 2 Chptr 4. Super quick , very little effort. But dont forget to shoot something with these relics as it will apply towards the other relic achievement "Relics of the Past".
I went back in after compaign was complete and set it on Beginner. For this achievement specifically, you will be picking up the Longshot Relic, the Lancer Relic and the Boltok relic because u need to equip all 3 at once, ONE HAS TO BE A PISTOL, obviously.
Load Act2, Chapter 4. Go past the Outsider camp on your right, keep going down path to your skiff.
Go in the direction the Skiff is pointing (briefly SW), bearing left, hug that cliffside on the left, going around it completely like a U turn, the buried traincars will be on your right. You will drop down onto the frozen lake and will see the red boathouse immediately to your left. The LONGSHOT RELIC is inside the boathouse on a table to the right as u enter.
For the next two, you have to open the Old Cog Wall. To find Old Cog Wall, just continue on east past the boathouse, hugging that cliff wall on the left, and it's right there.
RETRO LANDER RELIC:
Once u get thru the Old Cog Wall on your skiff, hug that cliff wall on the left again and see that tiny rock island with a circle of ice around it? Just before it on the left is a rock formation, the one with trees? Stop in front of that one, get off the skiff and face it. Your RETRO LANCER RELIC is at the bass of the tree on the right.
This one was hard for me even tho there are plenty of guides. I couldn't seem to find it using any of the directions in the written guides, and is the main reason I wrote this solution.
Find the East Tower SUBSTATION by heading east from that last Relic pick up. Go under the square shape ice bridge (the one that leads to abandoned Mine). You will see the substation in front of you. Once there, turn left and go thru the ice tunnel. Hug the ice wall on your left and follow it around. (Get fairly close to Condor Crash site but dont go under ice bridge). Just before you would go under bridge, you will see the Red Gears omen on the ice wall on the left and the Boltok Relic is on the ground just past it on right.