@K001aidD0g3z
I don't think that you'll find it of much help having a partner (or two) as it requires that you can communicate with one another. The game is tougher and if yr partner has been knocked out, the game gets really difficult. I'd recommend that you look up how to get through a session by searching YouTube. There are enough of them around (see attached link: Tips on Infinity Time). In addition you might want to consult the Plants vs. Zombies Wiki web-site using the attached link:Infinity Time
If it's any consolation. I had great difficulty when trying to get past stage 9 in Infinity Time 2 years ago and thought that I'd never get my 20-25.000 shards in order to get a legendary item, but now it's a question of whether I can be bothered to use that amount of time on the challenge (I got to stage 41 six months or so ago after more than an hour or so of gaming, but got knocked out in a moment's loss of concentration when thinking of whether to stop or carry on - That's not to say that I wouldn't get knocked out later as the game gets progressivley harder to get through).
P.S. I'd recommend completing the challenge by starting off using the cat (Zombie), as it is easier to target the mega-gnomes from behind cover, but it will mean that you will only be able to get Zombie legendary items. When you have got a feel for the challenge use the Triceratops (Plant) in order to get plant legendaries.