Seeing as there is no solution for this, i think it is a good idea to share what i found out.
Basically, community challenges are goals that are reached by the whole community of PVZ 2 players all over the world. There is no more than one active at the same time and it can range from killing enemies in a certain way to simply finish a certain amount of matches.
Note that as soon as the challenge is finished or the timelimit is met, the reward will be shared with the comunity, ranging as follows
Yellow - Blue - Red
Each stage has higher requirements but also better rewards.
For example, for 250.00 quests there is a gold reward, as for 750.000 and 1.250.000 there is a special community pack for you in store.
To get this achievement you need to meet the requirements to contribute to the great total.
The current challenge for example had 5 revives required to qualify for the challenge. These requirements need to be met BEFORE the grand total of the challenge is reached, so your best bet is to get these on the very first day. However, this is a little bit glitched and many people reported getting progress towards the achievement even though they did only later contribute to those.
Well, luckily the publisher really does a great job to take care of his customers, so here is a link to the publishers page.http://www.pvzgw2.com/events-calendar
As a short sidenote, there seems to be a blackmarket trader as well in this game, which appears as well as Xur from destiny on a regular base (i am sure it is no coincidence that it is spelled Xur backwards).
You can find this guy in the sewers and he sells sometimes useless, sometimes interesting stuff but for a really high price.
If you want to leave a negative, be at least fair and state why so i can improve the solution. Since this solution is meant to help and english is NOT my mother tongue, there might be some grammar errors, so be kind.
If this solution gets downvoted too much, i will delete it for good!