Warface

To reach rank 15, you need 53,000XP.

To reach rank 15, you’re going to have to dedicate quite a lot of time to leveling up.

There’s no effective way to level up aside from buying XP booster packs from the in game store. So just progress at your own pace and play whatever game mode is the most comfortable for you.

Personally I have found the co-op missions the best source of points and completing them everyday for the daily rewards.

Some tips for the co-op missions:

Know the mission

Missions are repeatable: play the mission several times and familiarize yourself with the map, available routes, NPC spawn locations, scripted events, ambush spots and try to find tactical positions for each of the 4 classes.
Know your team.

It’s best if you’re playing with someone you have played with before, have voice chat to coordinate team actions, and so on. If you don't have that, take some time to figure out your teammates – do they seem to know what they are doing? Do they like to rush or do they take time to heal and restock? Are they lone wolfs or do they try to work as a team? Keep in mind that the harder co-op missions are just about impossible to complete without some sort of teamwork.
Know the situation.

You can know what the mission is down to every single rock, but you can't predict what will happen. Check the actual situation before going forward, avoid tunnel vision, check your six, etc. Charging into a group of riot shield npcs is a bad time to realize that your team has been picked off by snipers.
Stick together.

Stay with the team! There are times when it’s advantageous to split up (fighting the Heavy Gunner for instance), but even then err on the side of caution. Finding yourself fighting 4 or 5 Blackwoods alone is not pleasant, or worse yet going alone against Riot-Shield Soldier! Staying together ensures that you will always have at least 1 person backing you up, in case you get wounded, need ammo, a boost up, or (continuing with the example) killing a Riot-Shielder.

Make sure there is at least 1 person covering you before you push forward!
Cover is available, use it.

The mission maps provide a lot of cover – use it. Keep in mind not all cover works the same: wooden planks will make it harder for the enemy to target you but bullets still go straight through it so you may still take damage; a burned out car might provide a good cover if you kneel behind it, but a sniper on a higher ground might still have an right angle to hit you.

Get out from under the fire. It should be your #1 priority to get to cover if you're being shot at; furthermore, don't try to shoot back. Chances are you'll miss, and in the time it took you to shoot and get killed, you could have slid into cover. Once you get to cover, stay there! You might not have a clear shot at the enemy from your position – let your teammates to take care of it, or consider the tactical situation and move to a better position… but moving out into the open will just get you killed.

If the cover is not working – move. It is not uncommon to see a player stay behind a wooden wall even though still taking damage. Or for a player to be hit by a Blackwood sniper once every few seconds but remain in place relying on the medic to heal him between the hits. This is a horrible "tactic", as it is neither helping your teammates nor disposing of the enemy.

And everything above is especially important during the defensive missions, whether you are defending against the waves of soldiers or say a gunship. You have to remember that important thing isn’t how many people you’ve killed, it’s how many people failed to kill you that matters; Hide, use cover, don’t stray too far from the team, cover your teammates, and wait for the clear shot. Ultimately, it is more beneficial to the team for the players to stay alive, not to kill as many Blackwood as possible before they die.

Note : Some of the mech missions give you cover such as burned out cars. If you hide behind one of these during the mech's rocket barrage, you may find yourself without cover soon, because the mech can destroy these barriers.
Advice for each Individual Class.

Medic

Don’t underestimate your handgun, even the most basic ones have surprising range and considerable accuracy. Handguns might lack stopping power, but you can still take out snipers at an appreciable distance with your sidearm.
Defibrillator can be used as a melee weapon – it is slow, but very powerful. Most importantly, defibrillators deal massive amounts of damage to Heavy Gunners: wait until the Juggernaut begins to reload, run up to it and shock. It will work regardless of whether you attack from the back or not, but it will deliver a lot more damage if you apply shock directly to power unit.
Shotgun – Absolutely devastating at close range, it can easily dispose of enemies within a few meters. It might feel counter intuitive, but you might actually want to charge toward the enemy (dive and slide if necessary to avoid some of the fire) to deliver the shot point blank. Keep in mind most shotguns are slow firing and very slow to reload so try to keep the weapon fully loaded, reloading in the middle of the fight cost valuable seconds and might cost you your life, so use time downtime to reload. On the other hand remember that most shotguns reload one slug at a time and can be fired even in the middle of reloading! Use this to your advantage!
Recognize the lost causes – one thing you have to accept as a medic is that you can’t save everyone, and some aren’t worth saving either. You won’t help anyone by dying yourself, so learn to recognize lost causes. If teammate dies in the open in a spot covered by two enemy snipers and a machine gun emplacement – trying to resurrect him will simply add your body on top of his. Leave him; you will serve your squad better by healing those who know how to use cover.
Be prepared to focus on healing. Medics primary role at all times is to keep the squad alive and healthy, but it's twice as true on defensive and safari missions. It is possible and even likely that in such missions you'll never get a chance to deal any damage at all. However without a skilled and dedicated medic such missions inevitably fail.

Rifleman

Prone will help steady aim and increase accuracy especially for LMGs, don't forget to slide into it! Remember to try to corner hug if not prone.
Don't be afraid to spray and pray. You can spare the ammo. In PvE missions, stray bullets from your spray can actually wound/drop enemies. (Suppressing Fire)
Refill teammates ammo when they need it! No one likes the rifleman who is too busy running ahead solo to refill someones ammo.

Engineer

Always help the medic with armour or the medic will die easily.
Make sure to repair your other teammate's as well. Armour may not be as good as health, but it helps keep players alive longer than just health alone.
Use your claymore in choke points or near areas where you know enemies will pass through. This is extremely useful when clearing any potential threats before proceeding by allowing the enemies to move toward you first. This can work extremely well against heavy troopers, where they can easily one-shot kill a player in Hardcore Coop and decimate an entire team within seconds if not taken cared of quickly.

Sniper

Try to hang back when the team moves forward into battle. A good sniper will pick off enemies from a far and also save their team when being flanked in doing so.
Prioritize targets on high ground or far away such as Demomen, Spec-Ops,or other snipers. It is easier for you to pick them off than a scopeless teammate.
Use your pistol in mid/close range situations if need be. Going into scope on your sniper at mid/close range will only limit your FOV. If you run out of ammo in your pistol clip, switching back to your sniper and spraying from the hip can also work in a pinch.
In PvE - Go for headshots! One headshot is faster than a couple of body shots and won't eat through your ammo like spraying would. Also, spraying constantly will only nuture bad habits and won't make you any better at aiming.

Know Your Enemies

Get to know what sort of enemies would appear before heading into a match. Certain classes were made to eliminate certain types of enemies, so it is best to know the most efficient way of handling a situation. please refer to NPC Enemies for more information about the enemies you will be facing
Communicate With Your Team.

It is best to communicate with your team, either through normal chat or through quick chat. Quick chat is normally used for asking for a Medic to heal, a Engineer to give armour, or an Assault to give ammo. These requests can be found by pressing F1, then either pressing F1, F2, or F3, depending on what you need. F2, F3, and F4 are also other communication methods. These methods are to normally be used during heated battle. It is best to use the normal chat to discuss a strategy or specify a request. One good example is how to take down the Heavy Gunner, or telling your sniper about the Spec Ops or Demoman. Communication is key to success.

Source: Warface Wiki.