First you'll need to have a polarized Weapon, Companion or Warframe (and this achievement btw)
If you haven't got it yet, I've wrote a guide on how to get it.Solution for Polarize This in Warframe
So, once you have polarized that Weapon, Companion or Warframe, it's level will be reset to Unranked (lvl 0).
You only have to do again what you've done to that item before => Getting it to level 30.
I don't have much councils to make you level up fast, but I can council you on how make it easier.
1. First of all (I should have said that at the beginning of this guide, but...f*ck it ^^), to make the achievement go faster, I would recommend you to polarize a Weapon more than a Companion of a Warframe because of the exp needed.
To get a weapon to level 30, you need 450'000 Exp on it
To get a warframe/companion to level 30, you need 900'000 Exp on it
+ If you choose to reset your warframe, your health and shield will drop on base values, and you will not (unless you've got an Aura Mod) be able to have your so loved mods that increase your health and shield by awesome numbers !! : You will be around 100 shield and 100 health, so I think you wouldn't want to farm Earth to don't get killed easily.
2. To make it the FASTEST WAY POSSIBLE (but I personnaly consider it useless for the rest of the game because that's not something principal), you should choose a sentinel weapon, because I've found that this item is the item who levels up most (and only need to reach 450'000 Exp), but as I said, if you are also enjoying the game, the Forma you would use for your polarization could be invested in a better way.
3. If you don't wan't to spend a Forma on a stupid weapon that you don't even control, and as I said the weapons were the best choice for this achievement, the best weapon type is : Melee Weapon. Why ? Two reasons :
First, you always use it in close combat, which occurs very often, and it's the fastest weapon to reach level 30 between your first, your secondary and your melee.
Second, you can equip a Stance mod to possibly equip mods even if your weapon is level 0 (don't forget that a fully upgraded Stance equipped with the correct polarity in the Stance slot gives you 10 in bonus drain)
+ Don't forget to select an useful polarity to quickly have mods equipped on your weapon.
4. You can potato (yeah, I know this sentence doesn't mean anything, I just wanted to make you laugh...), yup, you can use a "potato" also known as an Orokin Reactor, or the "blue potato" aka an Orokin Catalyst.
Reactors are more rare than Catalysts (even though they are rare)
Reactors equip to Warframes/Companions, and Catalysts to weapons.
What do they do ? They double the amount of drain points you can have. (Not counting the Stance). A weapon at level 2 (with a fully upgraded Stance + good polarity) will have 14 drain place, so at level 2, you can equip 2-3 mods easily.
The math is simple : (2 x Level) + Stance (with the polarity bonus), so a lvl 30 weapon + good Stance will reach 70 in drain place, which is pretty cool, because mods can easily get over 10 of drain, so you will then have a BADASS GEAR.
5. Yup, I've to talk about it, but you don't have to use necessarily : Affinity boosters (Double Exp).
I've completed every cheevo in this game just by having fun in it, and I never needed any affinity booster, so it will probably be useless to buy one.
+ Some gamers have reported bugs for other achievements where you need to reach level 30 with some weapons when using affinity booster
I hope this guide will help you, I hope your eyes didn't burn in reading my guide as English isn't my mother tongue, I hope everything is clear about every point, I stay open to any comments about it :)
Hope you'll have a great time in this awesome F2P game :)