Mass Effect

It'll help to play the game on the hardest difficulty (Insane/Insanity). Also, whenever there are large groups of enemies or a single, large enemy (Like a Geth Colossus) attacking while you are in the Mako...get out of the Mako.
The experience you get from killing enemies with the Mako is 1/3rd of the full amount.
Fighting the larger Geth my seem daunting without the Mako, but it's very possible and not that hard. My main advice is just keep moving. Ammo that damages synthetic foes also helps.
This also works when fighting Thresher Maws, but I never bothered since they tend to be bothersome even while in the Mako.

I ended up getting this on my Insane playthrough while trying to stop The Reaper at the Citadel. To help, try going for other achievements (like the player ones) so the extra missions you do don't feel like a major hassle.

There is a time consuming but very easy glitch to help speed this along.

When on planet Therum for the first time (meaning this can be done with a pre-existing character on a second or third playthrough), activate the mining laser (the code is AXBY) and then save as soon as you receive experience.

Then, load the save you just made. You'll notice you gained more experience. Save again, as quickly as you can, and reload the new save. Repeat as many times as desired.

The experience you gain scales with level, so this is faster than just killing hordes of enemies.

Therum is the planet you rescue Liara T'Soni from, and is located in the Knossos system in the Artemis Tau cluster.

Good luck!

This achievement is extremely time consuming. It took me 3 and a half playthroughs to get but it depends how many sidequests you do. Note that I did get the ally achievements while going for this so I did play through a ton of sidequests. 3 playthroughs is a realistic number for this. The main tip that I can give is to get out of the mako whenever in combat. You get 3x more experience which makes things much faster. Also, to get some quick and easy xp, talk to people on your crew and skip through the conversations. You may not get that much xp but it is quick and painless (I do recommend that during your first playthrough, you pay attention to what everyone says as the dialogue is excellent).

Please, keep in mind that as the experience is scaled based on your level, this method will be far less effective with high level characters. (One of the fastest ways from fifty-nine to sixty is probably the experience boost from becoming a spectre at the beginning of the game).

The following approaches exploit a glitch in the save system which allows the player to receive experience after loading a previously saved game file. There are several areas where saving and reloading [that same] game file resets the particular trigger intended to reward experience points. Deleting the autosave file places your most recent save at the top of the "Load" list, thus making it the default selection. Saving the extra keystroke will allow you to complete an entire save and load cycle in under thirty seconds. Saving to a new file each time will save you an additional motion and allow you a plethora of backups but mind the amount of free space on your harddrive as the files can be large

*Found in the Prothean ruins located in Artemis Tau / Knossos / Therum. After clearing the cavern of Geth the player activates the mining laser (combination: A, X, B, Y) and travels up an elevator to the floor on which Liara is imprisoned. The player will receive experience points [scaled to increase relative to your current level] after stepping off the elevator platform and onto the blue tile. Save immediately into a new slot (in case you need a prior load for backup purposes)-- it is critical to save before speaking with Liara. After the save is complete, proceed to the "Load" menu and select the file you just created. You will receive experience again upon load completion. Save again, then load. Rinse and repeat.

*A terminal inside the Feros freighter can be hacked after each save, load cycle.

*The lock on the sewer door in the Virmire base can be picked after each save, load cycle.

*The assault on the Citadel forces the player to exit the elevator. There are several hallways in following section where this method can be used to exploit the high respawn rate of Geth Destroyers.


//Please, feel free to add comments or make suggestions so I may update this!

[my writing; also posted to Mass Effect Wiki]

I had to play through twice to get this, and like everyone else says, get out of the mako, but remember LOOK EVERYWHERE, there are some easy exp drops on the Normandy and even in the Wards on the Citadel (note: at the Med Bay, look out over the city, also further down the walk where you meet the reporter. There are some on the Normandy, go into the bay and look at the mako, or out the pilot's viewport next to Joker.) Little bits like this help because they cost NO time.

This took me two and a bit playthroughs on insanity difficulty to get. Could be done in two playthroughs but I wasn't going to fight thresher maws.
Make sure you get out of the mako on planets to kill enemies as the experience is reduced by about a third when inside. For tough enemies like bigger geth and turrets do as much damage to them without killing them, use turret instead of main gun when health lower. Then get out and shoot a few shots to kill them.
Get every bit of codex experience you can pick up around the normandy and elsewhere. Talk to every crewmate and ask about everything, protheans, reapers when in conversation.