finally I've beat challenge tree farm
It was tougher than I imagined it to be, so I decided to write it here in case someone also struggles. Took me 2 days to figure out the strategy. At first I thought of resource converter (my idea was converter+condinsator) but couldn't make it work, then I tried probe for good tree positoniong, Undeground station (my favorite gadget in game) for revealing resources or minimap, hammer to last for more turn while trees to grow and even rockets because unupgraded hammer sucks. Trees just don't grow fast enough to make it work as I had to defend with 0-1 upgrade for far too many turns
Finally something clicked and I had an idea to plant trees on every cobalt I find and mine all other resources right away (i did plant 2 water trees midgame), then convert cobalt into iron every turn. This way cobalt kept me alive, water was used to increase cobalt-iron rate and soon I was swimming in Iron. I used Artilery dome (not that important) and drones to collect cobalt.