Mass Effect

You get the sentinel, Kaiden, from the start. Keep him in your party to do all the missions and about 40-45 assignments. You should unlock this achievements even before finishing the playthrough since both Kaiden and Ashley are in your party during which you have already done several missions before picking up other potential party members.

For missions see http://masseffect.wikia.com/wiki/Missions. For assignments, see http://masseffect.wikia.com/wiki/Assignments.

Because Kaiden is a sentinel class with both biotic and tech abilities you'll want to balance your party out. You should go for this achievement when you're playing either strictly as a biotic or tech, or are simply a soldier class.

*** Spoiler Alert ***

Toward the end of Virmire you'll need to make a choice to save either Kaiden or Ashley, the two humans originally in your party. Choose Kaiden because you cannot save both.

According to Mass Effect Wiki, you unlock this achievement by completing 45 assignments with Kaiden out of the disc's 73. Each DLC adds 3 to that list, making for a max of 79.

1) You do not need the character in your party to count toward your total, you just need him there when the assignment is marked "completed"
2) YES, DLC assignments can count, and are much easier than some in game assignments.
3) I suggest using Ashley or Tali as your other party member, since they both require 45 assignments complete, so you can kill two birds with one stone.
4) When you earn this, all characters will have recharge time for Lift and Throw reduced by 10%

ACHIEVEMENTS
http://masseffect.wikia.com/wiki/Achievements#Squadmates
ASSIGNMENTS
http://masseffect.wikia.com/wiki/Assignments

During my 2nd playthrough (New Game+, Infiltrator, currently Level 55) I got the Completionist and Soldier Ally at the same time, with Sentinel Ally achievement one assignment before. Below are the missions and assignments I completed to get these achievements (I had Kaiden and Ashley in my squad as soon as possible during Eden Prime):

Main Missions (15)
Prologue: Find the Beacon
Citadel: Expose Saren
Citadel: Garrus
Citadel: Wrex
Find Liara T'Soni
Noveria: Death of a Matriarch
Noveria: Leave Port Hanshan
Noveria: Lorik Qui'in
Noveria: Peak 15
Noveria: Reactor Repair
Noveria: Reconnect Landlines
Noveria: Contamination
Noveria: Rift Station
Noveria: Quarantine
Noveria: The Hot Labs

Assignments (41)
Citadel: Old Friends
Citadel: Asari Consort
Citadel: Doctor Michel is Safe
Citadel: Homecoming
Citadel: Jahleed's Fears
Citadel: Presidium Prophet
Citadel: Reporter's Request
Citadel: Rita's Sister is Safe
Citadel: Scan the Keepers
Citadel: Schells the Gambler
Citadel: Signal Tracking
Citadel: The Fan
Citadel: Xeltan Convinced
UNC: Privateers
Citadel: Family Matter
Citadel: Planting a Bug
Citadel: Snap Inspection
Citadel: The Fourth Estate
Feros: Geth in the Tunnels
Feros: Power Cells
Feros: Varren Meat
Feros: Water Restoration
Noveria: Espionage
Noveria: Smuggling
UNC: Asari Diplomacy
Tali and the Geth
UNC: Lost Module
UNC: Cerberus
UNC: Hades' Dogs
UNC: Rogue VI
UNC: Geth Incursions
UNC: Listening Post Alpha
UNC: Listening Post Theta
UNC: Depot Sigma-23
UNC: Lost Freighter
UNC: Distress Call
UNC: Major Kyle
UNC: Missing Marines
UNC: Hostile Takeover
UNC: Missing Survey Team
UNC: Hostage

I used this checklist: http://masseffect.shockfront.net/?page_id=2 to keep track of my progress, very useful.

I really thought this achievement was bugged but it finally dropped. This actually popped before completionist for me. I ended up doing the main missions up to right before Virmire (15 missions). I did 48 assignments when this finally popped.

I visited every star system possible to pick up every assignment and did every assignment on the Citadel. Kaiden was in my party for every mission and assignment.