Easy XP:
Play on the Security Tower map in Freeplay Mode, with 1-on-1 settings (Team Size should be set to 1, as should Max Team Size). Start as a Resistance member of the Soldier class. Blow the door to access the next mission objective, then change to the Operative class. Head toward the safe you are supposed to hack, and make sure not to be killed by the security agent. When that security agent/engineer tries to remove your hack box, position yourself on the opposite side of the glass. If you're positioned properly, your adversary will spray ineffectively and you will gain XP, due to the "hacking" and "guarding" functions you are performing. As long as you aren't positioned too close to the safe, you'll be stuck in a stalemate with your opponent and will earn XP the entire time. Then when you're ready, move closer to the safe to complete the mission and receive your points.

Using Heavy Guns on Smaller Body Types:
To use a heavy gun with a small body type, first, change to a heavy body type.
Choose a gun (make sure it's exactly how you want it as you cannot edit it
afterward). Pick any gun of any weight. Now do the same for your secondary. Now
make it your default weapon and go change back into a medium/light body type.
Your default guns will stay the same, but your body will not, thus, when you
play matches, you can have a light body type with a gatling gun.

Experience Requirements for Levels and Ranks:
You can reach new Ranks / Levels by acquiring the XP amounts shown below.
Rank 1
Level 1 - 1,000
Level 2 - 2,000
Level 3 - 3,000
Level 4 - 4,000
Rank 2
Level 5 - 5,250
Level 6 - 6,500
Level 7 - 7,750
Level 8 - 9,000
Level 9 - 10,250
Rank 3
Level 10 - 12,000
Level 11 - 13,750
Level 12 - 15,500
Level 13 - 17,250
Level 14 - 19,000
Rank 4
Level 15 - 21,750
Level 16 - 24,500
Level 17 - 27,250
Level 18 - 30,000
Level 19 - 32,750
Rank 5
Level 20 - 37,500

Hidden Message:
On the mission where the Resistance is trying to steal the airplane, at the terminal they are supposed to hack there is a message warning you of robots taking over the world.

Steam Achievements:

I live... again!
Objective: Revive yourself

The start of something big
Objective: Win any mission, whether campaign or What-If

Oh I'm sorry, was that yours?
Objective: Capture an enemy's Command Post

Use the wheel, earn more XP
Objective: Complete an objective after first selecting it on the Objective Wheel

You're going places, kid!
Objective: Reach Rank 2

Not over till the fat lady sings!
Objective: Take down an enemy with gunfire while knocked down

Well done!
Objective: Complete your first 1 Star Challenge

No I insist, you take it
Objective: Use the last of your Supplies to refill a teammate's ammo rather than your own

You shall not pass!
Objective: While on defense, prevent the attackers from completing their first objective

Objective: Detonate a HE Charge

Who's bad?
Objective: Complete all 1 Star Challenges

I think I know a shortcut
Objective: Open a shortcut for your team

That's how you win a match
Objective: While on defense, take down an attacker who's completing a Primary Objective

You can place another mine now
Objective: Take down an enemy with a mine

You've escaped the Ark
Objective: Win all main missions of the Resistance campaign (not including What-If missions)

Smart decisions win battles
Objective: Attempt to Revive an objective-class teammate over a non-objective teammate near a Primary Objective

Very well done indeed!
Objective: Complete your first 3 Star Challenge

Cut 'em off at the pass
Objective: Close an enemy team's shortcut

Was it the red or the blue wire?
Objective: Disarm an HE charge

You've saved the Ark
Objective: Win all main missions of the Security campaign (not including What-If missions)

They never knew what hit them
Objective: While on offense, win the match in less than 30% of the time limit.

Viva la revolution!
Objective: Win every Resistance campaign mission, including What-If missions

Not so sneaky now, are you?
Objective: Reveal an enemy in disguise

To serve and protect
Objective: Win every Security campaign mission, including What-If missions

Great shot kid! One in a million
Objective: Take down an enemy by shooting a grenade

The story has just begun
Objective: Win both story campaigns (not including What-If missions)

Pump up the volume!
Objective: Upgrade your team's command post

Well that was educational
Objective: Collect all Audio Logs

King of the world!
Objective: Complete all 3 Star Challenges

Time to start a new character
Objective: Reach Rank 5

'Tis better to give than receive
Objective: As a Medic, using the Transfer Supplies ability, give the last of your Supplies away

A bit of a headache
Objective: Take down an enemy with a Cortex Bomb

That mine you found? Disarmed!
Objective: Spot a mine which is later defused by another Engineer

Objective: Complete an Operative Primary Objective within 5 seconds of breaking disguise

Tough as nails
Objective: Win all storyline campaign missions (not including What-If missions) playing either Online Versus, or in Hard mode

It's a trap!
Objective: Take down an enemy with a Satchel Charge