The main thing that determines success or not in level 858 is your ability to open up the meshed blocks in the bottom two rows as soon as possible.
For this to happen it is easier if you have two or three in the same color, beside each other that you can open first, (if you don't see this when you first open the game, it might be a good time to quit and try again)
Pets drop randomly in this level, but there is a limit of four on board at a time and only one per column.
Try to open up the board and work on dropping as many Pets at the same time as you can.
Don't be afraid to release the virus and let it clear under some Pets while you work on others.