At first, the Foxtrot Ranger doesn't look like much when it comes to hardware. In addition to the standard rifle loadout, there is an option with boarding shotgun, a sniper rifle one and a light grenade launcher (plus the rifle).
The LGL is the interesting choice here. It allows for speculative fire from the midfield, with potential to dislodge well placed ODD/TO Camo snipers on a -3 BS check.
The boarding shotgun is a potentially deadly option in midfield, too. However, keep in mind that you will be needing specialists, so the forward observer loadout is definitely worth a look!
All but the LGL loadout also come with antipersonnel mines. This is a nice little transition to, arguably, the Foxtrot's most dangerous weapon: Efficiency! Yes, while the Ranger might look very bland at first, he fills an important spot in Ariadna's ranks: He is your go-to midfield option when you are short on points or you just want the bare minimum.
He brings all the tools he/she needs for the job at a super-competitive price in points. Even a rifle is deadly when used in good range and especially with surprise shot - easy to do for the Foxtrot.
Also consider the option to delay the opponent with cleverly placed mines!
The high physique value of 13, coupled with mimetism, is pretty much all that you can count on once you are discovered. Since armour and BTS are at 0, you should consider VERY careful placement and staying in camouflage most of the time.
Even as basic as he might be, the Foxtrot can still reliably pull of the classics.
Coupled with other infiltrators and hopefully having the first turn, you can use coordinated orders to place a number of mines and then re-cloak again.
Sneaking up on enemy snipers/missile launchers/etc. and surprising them with a burst from a good position should also be business as usual.
Finally, if you chose the LGL option, you can put some good pressure on hidden enemies in/near their DZ. Also remember that you can catch camo markers in a grenade blast, as long as there is another non-camo model near them.
As soon as they are discovered, you should have no problems in dealing with the Foxtrot. For better chances regarding firefights, consider using MSVs.
There will be room in a list for a Foxtrot 99% of the time - the low price is one of their biggest strengths! If you miss that extra points for bringing a Chasseur, there is always the option for a Foxtrot. Usually, I bring one in addition to Uxía, which gives me 2 midfield specialists.
If you have the first turn, the Foxtrot FO can be placed to take midfield objectives. In case of your opponent going first, consider to either hold back a little or place the Ranger in hard to access places (like prone on the roof of a 2-storey building).
Active Turn Role
As said before, taking objectives and forcing favorable firefights on your opponent are the standard role of the Ranger.
Always remember your mines, too. If you can go for a few cheerleaders standing clustered together, a mine can be absolutely devastating!
Since the Foxtrot lacks defensive tools, laying low and hiding should be your go-to ARO options.