UPDATE: As operation 5, classes and characters has been divided, the rest of the achievements show the requiered class for each achievement, but you need to use the characters for this one, not their previous classes.
This achievement shouldn't really need a guide but there's some pointers. It requires at least 2 people.
Required Character: Baird (Released on Op 2)
Required Character: Paduk (Released on Op 4)
Required Mode: Horde Frenzy (Beat waves 1 to 12 on a single sitting)
Aprox Time: 25-40 minutes
As of Operation 4, you no longer need to unlock the characters with Totems/Challenges, you just need to get 1 skin of the selected character to unlock it, wich can be done in both Tour of Duty (IV for Paduk) or via the gears store (with Gears coins), so just search for a skin of the character and purchase it (Search for the 2,000 coins options).
If you have a team that haves your back, just launch Horde Frenzy (12 waves) on the desired difficulty and map and run towards it normally. If it's only you and your teammate, search for a Beginner match and beat the map, it shouldn't be too difficult, everyone on the team can get the achievement.
On Frenzy, all players start with 10,000 credits, so make sure to deposit and reinforce your base. As soon as the match starts grab the fabricator and drop it, two energy tabs will appear, move the fabricator nearby and start building. As i said before, it's a pretty simple achievement, specially if you play on the lower difficulties.
Baird and Paduk are pretty useful characters once they have leveled up but that will require some time, Baird's speciality are long repairs and his Db, while Paduk's fear and bleed damage can do a lot of damage.
You shouldn't have any issue with any map on beginner.