The lore behind our clones and New Eden defies this as a valid scenario, except in the event of a "Spec Ops" type mission where a VERY small number of clones can be sent/hacked/somehow accessed behind enemy lines as a one-off to try and sabotage something/assassinate a non-immortal.

I can think of a few ways to justify this kind of scenario within the lore.

1. Extremely advanced Rogue Drone infestation across a whole planet. All infrastructure has been destroyed or subverted by the machines already, and attempts to drop CRUs in the area have failed, as have attempts to move in a proper assault force. The Skyfire Battery in a certain key district needs to be disabled before a proper assault can be launched anywhere on-world, and the Drones also have control of several ships, which are patrolling the area and making it difficult for our boys in space as well. Since even CRUs have been tracked by the Drones and intercepted before landing, we're sending in something smaller. A CovOps vessel will be making a low-orbit drop of a handful of Mercs - all it can carry. Even Uplinks are of no value, as we can't maintain a large enough vessel in proximity with the planet to provide extra clones.

2. PvE contract against automated security systems (possibly including Drones, but not Rogue ones). The facility is manned, but when the Merc assault arrives, the base goes into lockdown as it's obvious none of the non-immortal defenders are equipped to handle the situation. There could be locations where, if you can locate where the people in the base are, you can hack a security door and force them to surrender, disabling the defenses in an area (or turning some of the defenses into allies) because they know they can't respawn. There's the risk of acting outside the game's current violence levels (and thus rating) with this if you fight any unaugmented humans though.

3. PvP scenarios with one team having no respawns. This would have to be objective-based. The attackers would be several squads of Mercs dropped behind enemy lines to sabotage something or destroy several targets of opportunity. This possibility has a couple of ways it could be balanced.

3.a) The attackers with limited numbers could have a larger number of Mercs available, while the defenders have more clones to use. This could be explained as (for example) a startup clone manuufacturing Corp with limited funds to hire Mercs with, but the ability to provide clones for those who take the defense contract, while their more powerful rivals can afford more Mercs, but are unable to get their own clones on-world to reinforce when the attackers go down. Basically, it would be 16 vs. 4 or 16 vs. 8 or some other situation, but the smaller team would be able to revive when they go down. There would need to be a single objective, or several that need to be taken (or destroyed) in sequence, so the defenders can concentrate on a single area and position themselves for various ambushes on the attackers.

3.b) Equal numbers, but with multiple easily-completed objectives that can be attacked in any order. This way, the smaller force would have a chance to lure most of their opponents to one objective before taking another, or use deceit and misdirection to get someone to the objective before going down. If you knew you had the skill or gear advantage, the non-respawning team could just zerg all the objectives simultaneously, and eliminate/hack everything before the defenders have time to respawn.