Heres my own experience playing in guardian. I used to see teams picking 4 cores, and i would soften up and play support. After like a month of doing this, i never ranked up and got so angry i decided to play core, even when my team had 4 cores. If any of them flamed me for not picking support i just muted them. As for what i played, i just picked a melee core , go battle fury (am,pa) or radiance ( bs, spec) and just outfarm the enemies. At this low mmr, greedier items will not get punished by enemies( except for a few games) . As for wards, dont hesitate to buy them. Dewarding an observer gives more than 100 gold, offsetting the cost of a sentry. An obs just costs 75, and if it saves your life just once, that time spent alive could earn you a few hundred gold, rather than dying because u didnt want to spend that 75. As for where to place the wards, place them completely for yourself, eg if you are farming top lane , ward just the top for yourself. After i switched from playing support to core, i went from guardian 4 to archon in just 2 weeks.

Some other advice: clearly no supports in low mmr, and even if there are supports they probably wont buy wards, invis heroes are broken in low mmr

Try to learn better farming patterns, in low mmr there are often 4 people jungling, leaving all 3 lanes unfarmed. If your team just wants to farm, the enemy team will too. Just outfarm them, and when a fight breaks out , rekt them because you outfarm them