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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.