gotcha speaking of ambush how much should I try to use or relay on stealth sniper ?
Ambush works the easiest with a sniper rifle. Other weapons limit you with their range, but this gun can shoot anywhere, so all you need to trigger Ambush is to finda dark spot while still being able to have visual on the enemy regardless of how far you are. In fact, your range protects you from their attacks, as other guns can't fire properly when being too far. Ambush also has another very improtant aspect, it drastically improves your aim. You should always try to start fights from a dark spot. Can place Bear Traps on lighted areas or even throw a Molotov to ensure the enemy is lid up.
Snipers are the most fun, but you really have to devote into them - not just by feats, but in practice. I fully agree with harperfan7 on this. The rifle should be mained. No need to use any other weapon, let alone two, and throwables should not be for dealing damage, just for utility. The more you kill with the rifle, the more you can shoot with it per round thanks to Shooting Spree. It also reduces cooldowns of Aim Shot. and with Snipe and Tranquilified, Future Oriented Temporal Increments, it becomes a weapon that will be very greatful to you for using it, returning AP in multiple ways and more frequently with each kill. Using anything else would just disrupt that.
Shooting Spree is essential (spec it fully before anything else, then spec Future Orientation). You also need Snipe with maxed Stealth. It deals enormous damage and since it's not crit-based like Aim Shot, it's also useful against enemies who are immune to critical hits. Try finishing turns with entering Stealth behind cover. Also here's where I would recommend Interloper. Not only it saves you 10AP each time you step out from cover while stealthed, but it tremendously boost your stealth in practice due to passing by enemies faster - so their "eye of awareness" doesn't fill up. Once you get it, you don't want to play without it. For this reason, I would recommend playing with 5 DEX and 7 AGI.
This is an older guide I did, and I would most probably switch something like Premeditation for Polymath due to the increased crafting difficulty introduced since, but even without the Metathermics, it contains some useful tips for sniper users.
https://stygiansoftware.com/forums/index.php?topic=10837.0