One thing to always keep in mind is that Rangers have the longest sight radius in the game, and firing range even longer than that. It is very much possible to damage or even kill a unit without having it see you, and thus, counterattack. And since they are rather fragile, it's best to try and make that happen.
However, their damage output is very low, since only one of the models in the squad is armed with the long rifle, the other ones carry swords (named "power swords" but dealing normal melee damage, not power melee) and shuriken pistols. Compared to a Guardian Squad, they will deal 90% as much DPS to heavy infantry, 60% to infantry and 40% to heroes.
Although the long rifle has a 1.5 setup/teardown time, they can fire once every 7 seconds, so unless it's done right before they fire, having their formation broken with knockback isn't much of a problem. It does, however, mean that they won't be dealing any damage to squads running away, especially if they're doing it via the Fall Back function.
Their sight and detection radius also makes them great scouts. It should also be noted that they have the best detection radius in the game, though not big enough to snipe infiltrated units out of their sight range. It will, however, allow them to get a jump on enemy infiltrators or plan in advance. It's usually best to send other units to fight a detected infiltrator and keep your rangers away though, as only other infiltrators they can consistently defeat 1 on 1 are Lootas and Scout Marines without a Sergant, and even then it's rather risky. Especially if they have a weapon upgrade.
A main use for Rangers is as a T1 anti-suppression unit. All T1 suppression squads (Devastators, Havocs, Lootas, Shuriken Cannon Teams, and Heavy Weapon Squads) have a sight radius of 40 and weapon range of 49, while Rangers have sight of 60 and their long rifle has a weapon range of 65. This means they can safely fire at them not only out of their weapon range, but even out of their sight. Moreover, their Kinetic Pulse ability forces the enemy to set up again. Useful if they decide to chase your position, since it gives you some time to get out of their sight range and start sniping again.
Although this allows them to pick off suppression squad members (especially stationary ones) safely, they will need a relatively large amount of time to do so. For example, Rangers need approximately 39 seconds (6 shots) to kill a devastator squad, whereas Stormboyz can do it in about 5.
For what it's worth, though, they are the cheapest anti-suppression unit in the game barring the Sentinel.
Cover and anti-garrisonEdit
The long rifle ignores light/heavy cover and garrison; only Eldar Energy Shields can defend against it. This allows them to whittle down fortified positions from extreme range and counter garrisoned units (especially suppression squads), although the Rangers will need a while to force off – or kill – the units.
The Rangers themselves are extremely vulnerable to melee, and if a dedicated melee unit can reach the Ranger squad, it is often dead. They should aim to stay at maximum range so they can relocate well before being caught up. Failing that, the Infiltrate ability may allow them to slip past melee troops.
The Kinetic Pulse ability, however, grants a single-use anti-melee option. This is most effective combined with other ranged units, allowing you to focus fire on the approaching melee unit while it gets back up, and possibly forcing it to retreat before engaging. The knockback can be combined with AoE attacks such as a Plasma Grenade or Immolate for more hurt.
In a tight spot, use Kinetic Pulse before retreating to compensate for the long rifle's teardown delay.
Rangers fire slowly and are very vulnerable, so fighting 1 on 1 against ranged units might be a bad idea. You can still outrange them and hit them out of their sight, but unlike melee units, they don't need to be very close to you to kill you. Thus, it's generally better to snipe what models you safely can, then either relocate (with Infiltrate if necessary) until the enemy finds another target or just Fall Back before losing models.
Here's a crazy little thing though, forcing melee might actually help you win some fights against ranged units, especially if they've already been damaged with your Long Rifle. This is because their melee DPS is actually higher than their ranged DPS (40.01 melee, 26.27 Pistol range, 21.33 Rifle range), whereas most ranged units are weaker in melee than they are at range. They can beat Guardians, Termagants, Scout Marines (again, without Sergants), and Guardsmen (with or without Sergants) before they can kill rangers even if we don't take any ranged damage dealt into account. It's a very risky tactic though, and you should probably Fall Back instead. Especially against Tactical Marines or Chaos Tactical Marines that have Melee Resistance, or against Shootas that are plain out good in melee.
Although attacking them may seem like a great idea, sniper weapons only deal 60% damage to commander armor, meaning it will take even longer for Rangers to kill one, and give the commander plenty of time to find you. More importantly though, commanders often have a way of getting to, or disabling your Rangers. Some are even resistant to Kinetic Pulse's knockback. It's still a viable option though, especially if the target is busy getting a control point, but they should be killing infantry/heavy infantry, or even better, suppression units.
Rangers deal insignificant damage to any sort of vehicle (except the Sentinel, of course, which has heavy infantry armor), but can make themselves useful with any kind of anti-vehicle unit with the Holo-field ability. Infiltrating a Brightlance Weapon Team so the opponent doesn't realize the source of damage until too late, or cloaking a Howling Banshee squad to ambush an enemy transport/tank from behind may catch your opponent completely off guard.
Infiltration can allow Rangers to approach and melee deployable Tarantula and Heavy Bolter suppression turrets, but their damage output is rather low, which gives the opponent plenty of time to defend the turret. A more effective strategy is to use Holo-field to move another unit – such as Howling Banshees – past the suppression arc so they can take the turret down faster.
When applying this strategy, note that the turrets have a detection radius of 15, meaning that any squad charging a turret head on will be detected and suppressed before reaching it. Use the infiltration effect to cut across the suppression arc diagonally and flank the turret instead.