It already is
First-level characters are squishy, and a Rogue with Surprise could single-handedly gank a caster on its first attack if it rolls well; 1d6 (eg, shortbow) + 1d6 Sneak Attack + Dex Mod (eg, 2) often sums to more health than a Sorceror possesses. The Fighter's burst damage depends on the exact build, but it's unlikely to do less damage than a first-level Rogue, especially with Action Surge.
My only firm opinion on the antagonist's build is that they focus on ranged combat rather than melee. Intelligent foes, which typically (but not always) includes players, will make short work of a low-level melee rogue, either through direct damage or crowd-control (eg, spells with Con saves). A ranged rogue still gets sneak attack on targets adjacent to its beefy minion with significantly less risk from melee damage-dealers.
Battle Master Fighter
In my experience, the best way to force players to be creative is to make them believe they're outclassed, regardless of whether that's mechanically true. There's a tricky balance between outright damage, versatility, and countering their tactics. Ideally, you want a Fighter who's dangerous on his own but also makes the Rogue more dangerous.
If you're just using the PHB, your fighter options are Battle Master, Eldritch Knight, and Champion:
- Eldritch Knight is arguably the most versatile, which is both a pro and a con in this case. It's better able to deal with PC variety / shenanigans, but also means they'll have a harder time creatively dealing with it, since they won't know its spell list. Moreover, many of the damaging spells on the Eldritch Knight's spell list are in supplementary books, so you won't have as many options for overwhelming force, which therefore means players are less likely to feel outclassed.
- Champion is both too binary at low levels and too dangerous. At level 1, a crit from a strong melee combatant has a good chance of killing them outright, which will make them feel outclassed, but also unlucky and resentful at having to immediately reroll.
- Battle Master gets four maneuvers early on, and while they don't measure up to spells later, they're quite useful at-level and are especially strong against low-level characters.
The maneuvers I'd consider for the purposes of working with the Rogue are:
- Bait and Switch: If the group manages to get close to the Rogue, the fighter can interpose itself and protect its comparatively-squishy boss. Alternatively, Maneuvering Attack, so the Fighter and Rogue don't necessarily have to be near each other.
- Distracting Strike: I'd call this mandatory for Rogue-enabling. Advantage means guaranteed Sneak Attack (in case a desired target's initiative falls between the Fighter's and the Rogue's), which makes the Rogue more powerful and incentivizes the party to deal with them individually.
- Feinting Attack: Advantage, which makes the Fighter scary individually, especially if it's picked up Great Weapon Master. Combine with Action Surge for more fear factor
- Trip Attack / Pushing Attack / Menacing Attack / Grappling Strike: All serve the same purpose of battlefield control. Your selection can be tailored to their party composition.
Otherwise, you don't need anything special
A fourth-level Fighter has up to two feats, depending on race, and even with a standard array can easily reach a Str of 18. A +4 damage modifier and Great Weapon Master probably makes it too deadly; the +14 base damage is more than most players' max health at level 1.
I'd recommend the Protection or Two-Weapon Fighting fighting styles; the former helps the Fighter better shut down would-be assailants of the Rogue, and the latter allows it to be dangerous to multiple targets in a single round.
If you're looking for Uncommon magical items, I'd suggest something more utility-focused so you don't one-shot your players: Oil of Slipperiness (battlefield control), Eversmoking Bottle (battlefield control, defensive for Rogue), or even Dust of Disappearance (escape mechanism).
Tactics are critical... to avoid a TPK
It's hard to predict DPR without build knowledge, but the Fighter could easily one-shot a caster with a (un)lucky roll. With a two-handed weapon or a critical, a Barbarian or Paladin will also be down, either "for the count" or "for good".