My best record with Malph was that the enemy Irelia wasn't level 6 by the time I'm level 9, simply because I'd run between her and her minion waves whenever she tries to leave tower (the reason she's gotten past level 3 was that I made a mistake and died in a gank where if I didn't I'd gotten double kill).
The way he harasses is that your shield makes you 'win' the trade. If your shield break then stay back until it's up, then back to QQQQ. If they don't break your shield then keep up the QQ.
If they're under tower, but doesn't have a good stun(near instant stun like Veigar's E or Panth's jump can be deadly here), then you can even run up, Q, then run back out while taking little damage because your shield should easily almost fully absorb a tower's shot(you'd take maybe 20-30 damage)
You should always be trying to last hit if it doesn't push the lane outward(in 1v2 against something like Urgot/Panth, the ones that can burst you down if you're not careful), and pretty much always last hit if possible in 1v1, 2v1. Unless they have an instant stun, you can even harass some champs right under their tower. Malph with no gold isn't scary.
Having low kills shouldn't matter so much, but if you want to increase your lethality then using Ignite is an option(REQ Ignite usually will kill anyone after 1-3 leading Qs).