so i been trying to switching to sniper (the bolt-action type, not Latron or Vectic) and i figure out that im suck at headshot, mostly when the enemy is moving, or when he's a Grineer, goodam Grineer with they oversize back.
so yeh...you guy have any tip how to aim properly that you guy wanna share (beside aiming for the head) ? 

The advice I usually get when dealing with snipers in shooter games is that you need to lead your target for a bit so that you can get used to their movement. Supposedly by doing this, you can take your shot knowing where they will be, which will help you get more hits.

Well, if you want to use a sniper and suck at sniper (like me), the I would suggest ME2 and ME3 as an infiltrator because it actually slows down time when you look through the scope and you actually get headshots quite frequently. That being said, playing ME2/3 actually helped me because I slowing kept speeding up the time until it was eventually real time and got used to how they moved. Obviously, things are a little different depending on the game, but with enough practice, it is possible.

Yes, leading the target is always prefered, but it is quite difficult. I would suggest hiding in cover and waiting for the mobs to run to cover and then just pick them off when they are not moving. If, however, you are like me and prefer to go guns a blazing, try putting your reticle (unscoped) on their head for practice and then zoom in to confirm you location before you take the shot. In my experience, the hardest part is moving from the normal FOV to the limited, scoped FOV and losing my bearings.

I guess it's just about practice? I remember i had the problem with hitting moving targets a few years back when i finally started playing shooters, but i don't have that feeling anymore.

I guess i would just recommend trying to get used to snipers in something like warframe using lanka, you have to lead the target but you don't have to worry about bullet drop or anything. I recently picked up Planetside 2 and sniping from over 300m was really annoying in the beginning.