The Renegade is probably one of the most obnoxious levels out there. Montana and Reyna actually would be a damn good pick, if whoever you’re playing it with knows that they’re a support. They’re in charge of keeping you shielded (and healed, always take the heals on her booster at level 4), while you’re in charge of dishing out the damage. Montana is a pretty good pick even for solo PvE, as he gets good damage reduction which synergizes well with health regen.
I’d suggest finding a character you are comfortable with and sticking with them for a few missions, trying to work out gear builds/strategies. There’s a fair few guides lying around here.
Edit: Some basic strategies for both characters below.
Montana: He gets about 20% (I think) damage reduction while Hailstorm is active. You should pretty much be activating it as soon as it comes off cooldown. Lumberjack Dash is really useful for stunning enemies with shields, like Thrall Brutes, making them much easier to hit. Try to use your ult when you are surrounded as the damage + Knockup is really useful in large groups. Helices I use are: 1.Left, 2. Middle, 3. Left, 4. Left, 5. Right, 6. Left, 7. Right, 8. Left, 9. Left, 10. Left. In terms of gear try to focus on Max Health, Health Regen and Attack Damage.
Reyna: Priority Target is useful for getting extra damage on enemies with large amounts of health, like bosses. The helix at level 2 where her secondary attack homes in on Priority Target is extremely useful. Her Overshield can be used effectively to rescue a near-death teammate, while also giving her a bit of survivability. You can either save the shield until you really need them, or you can fire them off at will - it works well either way. You can pop her ult over or near a defense objective, effectively protecting it from fire for a short period of time. Helices I use are: 1. Left, 2. Left, 3. Left, 4. Left, 5. Left, 6. Left, 7. Right, 8. Left, 9. Right, 10. Right. Gear would be Cooldown Reduction, Health Regen, and either Heal Power or Attack Damage, depending on whether you want to focus on support or DPS.
For The Renegade, try to get as many shards as you can. Build as many turrets/traps as you can in the last defense point, preferably during the countdown before the wave starts. Then you can sit inside right next to the point and sustain fire on the two choke points in the doorways. Thumper Turrets and Temporal Traps both will help slow enemies down, allowing you more time to finish them off. Enemies will occasionally drop in behind you, but this doesn’t happen often and you can usually just pick them off quickly or leave them to your turrets.