You should actually NOT use E to initiate if you can help it. Most enemy top lanes are going to be Melee, and so they have to get really close to last hit, same as you.
You only use your grab vs enemies like Ryze,Kennen and ranged harassers which are fairly rare. You use your grab as a kill-securer after youve engaged and they are running like the cowards they are.
You will max out your Q first so you can use your axe at medium range to do maximum damage and use it as harassment. Its relatively cheap manawise and does solid damage.
They will get used to getting hit with your Q and used to being close to you. So when they move to lasthit, you Q them as usual, then simply right click them and walk up and attack. Soon as it lands, you immediately use W to reset your attack timer and get in your Slowing boosted instant autoattack.
They now have 3 stacks on them and they are slowed while you have 5% bonus movespeed. The 2 second slow is sufficient to get in 1 extra autoattack. Ideally two but likely 1. The CD on your W is 8 seconds, minus 2(or 3 not sure) for the 2/3 stacks of bleed on the enemy. This is important.
So they have 4 or 5 stacks on them. You can either:
a) Grab them now and get 1 extra autoattack to get 5 stacks then Finish with your Execute.
b) If 5 stacks (you got off 2 autoattacks when they were slowed), go ahead and execute! It acts as a GapCloser and it going on CD isn't a big deal when you are toplane.
You can then wait a few sec, Q them, re-grab them, re-W slow them.
c) Wait for your W to refresh, THEN grab and re-slow them.
The danger with (c) is that your bleed counters may expire before you can grab and re-hit them which would totally hoze you.
You will learn thru experience the best way to go. (a) is the fastest if you can get the kill. (b) is good for maximum damage, though it puts your ULT on CD. (c) is really only viable once you have some additional CDR from Defensive/Shroud/Brutalizer/whatever.