Give it a focus sash, and use Endeavour. Assuming the opponent attacks, Sunkern will probably be taken down to its sash. Since Sunkern will almost certainly be slower it will then use Endeavour and take them down to 1HP as well.
Of course this is incredibly gimmicky, and is stopped by a lot of things - Burn, Poison, Sandstorm, Hail, Sun (because of Solar Power), multi-hit moves, entry hazards etc. You get the idea - anything that will make the Focus Sash useless will make this strategy useless, and you will have a dead Sunkern incredibly quickly.