The original author of one of the Xbox 360 solutions for this exact achievement, xI Alphamale Ix, has been gracious enough to share his solution for the Xbox One version of the game. His extensive guide can be seen in it's entirety below:
In Plants Vs Zombies: Garden Warfare, you level characters by completing challenges. The challenges don't get MUCH more difficult, but the number of challenges needed to increase each level constantly goes up.
You can skip any challenge using a Skip Challenge consumable. These are available in the Sticker Store. Your best bet at finding these are by purchasing the 10,000 coin packs; however, I chose to almost exclusively purchase the 20,000 coin packs because of the two character stickers included. Character stickers unlock new characters. These unlocked characters hold significant edges over their "standard" class counterpart. The challenges carry over for each character in the class. For example, if you get the Chomper to level 6 and then unlock the Vampiric Chomper, he'll still be level 6 with all the same challenges. 40,000 coin packs will automatically unlock a character, if you want to go that route, but 20,000 coin packs include 2 random character stickers anyway. Note that 1,000 coin packs do rarely contain a single Skip.
Skip Challenge consumables are VALUABLE. Don't waste them! You won't come across them a whole lot, even when opening high value packs. You'll probably see between 40 and 60 on your journey, depending on luck and where you spend your coins.
In earlier levels, there are very few challenges between levels. Every level adds more challenges that must be completed (or skipped) to reach the next level. The challenges are semi-random. There's no guarantee which challenges you will or won't receive. They do get progressively more difficult, but they never feel overwhelming. So, if you're a level 7 Cactus and want to skip a challenge, don't feel like you can't because you're saving it for higher levels. There are simply MORE challenges per level - there are 15 challenges during the last few levels before level 10. Most often, your task count will simply be higher in the later levels, with maybe a suffix added. I can't say for sure which will be harder to you, as they're often more about play style than anything.
Let's have a look at some of the challenges. The first thing you want to ask yourself is "WHAT MODE should I be in for this challenge?" This is very important, as some challenges are much more difficult in player vs player than Garden Ops (co-op). If a plant challenge doesn't have the word "Player" in it, it can be done in Garden Ops on Easy.
SUFFIXES
These will start showing up at the end of the tasks you're asked to do for each challenge to increase the difficulty. Here are the main ones:
...in a session
If something must be done in a session, it must be done in one game. Plant session challenges should quickly make you look toward Garden Ops, if possible. For the rest, you'll want to look toward G&G, since the sessions are typically longer. These start showing up around level 5.
...without getting KO'd
This means it must be done without dying, like get 3 primary weapon vanquishes without being KO'd. These show up in later levels. Sticking close to teammates should help with these. If this makes you uncomfortable, beware that they're coming in later levels if it's something you feel you'd need to skip.
Let's get into the challenges.
Use [ability] X times
Press the button. This one couldn't be easier. You unlock all the abilities by level 3.
Skip: NOT RECOMMENDED
Some challenges are very easy like this one, but not many. We won't detail all those. Also, the ones that are really easy like this do actually show up in late levels.
Use character for 30 minutes
This is just a cumulative amount of time you use the character class. No skill involved.
Skip: NOT RECOMMENDED
Vanquish [plant or zombie] X times
Just get kills. If this is a plant challenge, you can do it in Garden Ops, if it would be easier there. For example, waiting for 5 screen-door zombies would be a pain in Gardens and Graveyards, but it's pretty easy in Garden Ops, so do it there.
Skip: NOT RECOMMENDED
Vanquish [plant or zombie] X times with [ability]
If it doesn't say "Player," it can be done in Garden Ops on easy. This has a lot to do with play style. Vanquishing 3 Peashooters with Sprint Tackle sounds fun to me