I just did the Imbellus assessment one month ago.If you have already done your research on the matter, you won't experience any trouble completing the assessment. There are basically two types of game that you have to complete: the first one is eco-system building and the other one is about protecting a flower from invaders on a round by round basis.
While the second game was pretty easy to understand, managing to create an eco-system that is sustainable is a bit more complex. There is a lot of data and you don't really get the time to make sure that all the choices you make are correct.
My best advice would be to keep track of the time and to remain calm during the assesment. I don't know how much the test accounts for your overall performance but I didn't get any feedback on my performance. I believed that I failed the first game and still got invited to interviews later on. In a nutshell don't stress too much about the test, it will be fine.
Hope this helps, best of luck for your test !