The biting blade will get you strength 7 on average because you should normally get 3 hits because of the exarch's improved weapon skill.
His weapon skill has nothing to do with hitting vehicles. (recall, you hit automatically if the vehicle didn't move, half the time if it moved at combat and on sixes only if it moved at cruising. The only vehicle type for which that's not true is dreadnoughts.)
EDIT: Okay, OP. I'm going to put together some kind of a list based on what you have. It's probably not going to be amazing, but feel free to copy, steal, maim or otherwise use it
Okay, two choices for HQ, each of which depend on a different choice.
It all comes down to the wave serpent. You've only got one transport, so you need to think about what goes in it. You don't have enough dire avengers, and neither guardians nor rangers really need or want to be mech'd. So I figure you've got two good options. You could take a squad of scorpions with a farseer, or you could take a squad of warlocks all with destructor.
So, HQ: Farseer (witchblade, not singing spear) with Doom: 80 points
4 Warlocks (witchblades) and 3 Destructor - 130
I don't know which squad I like more.
Some points: they're both really deadly. The scorpions with farseer burn through enemies in cc, and aren't weak vs vehicles because the farseer has 3 S9 attacks on the charge (vs vehicles).
The warlocks do a triple heavy flamer, and then 12 WS5 attacks that wound on a 2+, which isn't shabby either. (And obviously, the warlocks burn through vehicles like parchment, with 12 S9 attacks on the charge). (For either army, you want to move into position, and then disembark next turn, so you can charge that same turn.) The scorpions, however, are a lot more survivable than the Warlocks (and for 5 points more, you can have fortune instead of doom. Use it turn one on your wave serpent and turboboost, then turn two on your scorpions and charge. Good times, and you'll likely put the fear of god in your opponent). (Aside: if you have your warlocks in a vehicle looking at, say, a squad of marines in front of a rhino, do this. Disembark your warlocks and move forward. Gauge the distance and see if you could hit the rhino with your flamers. If you think you can, then shoot the rhino, killing the marines as you do, then assault it, killing it.)
SO. Depending on what you pick there, I'd take:
6 Striking Scorpions, including Exarch with scorpion's claw - 135 (I made this points value up, don't have my codex nearby)
6 Striking Scorpions including Exarch with scorpion's claw and Shadowstrike - 150 (ditto)
Obviously, the first squad would be with the farseer in the transport, the second squad would be on foot with the warlocks in the transport.
Either way, that means: Wave Serpent, TL bl - 135
10 Guardians, EML - 100. This unit isn't great, but at low points levels they tend to be cheap and survivable. Stick them in cover, remember your weapon platform is Assault so you can move and shoot, and watch them slowly die (guardians don't tend to run, and you've got enough wounds to last ages. They tend to attract proportionally greater firepower, and the blast EML is pretty fun)
5 rangers - 95
Okay, two defensive troops choices maybe isn't great, think about this. Possibly, your wave serpent could come back and grab the guardians midgame.
At this point, I'd fill the rest of the points out with the Dark Reapers and/or the War Walker, depending on which of the above options you preferred.