If you are looking for an easy place to level any weapon, there is an infested survival mission on the north end of mars that starts at level 10. Even a level 1 weapon will do enough damage to power through for a minimum of five minutes. If you have a relatively powerful warframe, i would recommend that you unequip any other weapons you may have that are not max level, so as to nkt spread your bonus xp across all of them. Between killing and bonus xp, i was aeveraging two levels per 10 minute run starting at level 20 on my Furax. Once you hit level ten, it is worth it to start putting in mods to speed up your killing just a bit. Doing this, you can level up rather quickly. If you are going for your 3 max level achievement, you can level up to 29 with completion, and then get your xp bar close to 30 and just abort mission. This will save your xp close to finishing. Then you can repeat the mission, level your weapon up to 30 by killing, and force close the game as soon as you see your achievement gain progression. As of 8/8/18, this method still cheeses the achievement.