How do I fight Alghouls effectively?

My technique for this monster. I rely on dodging and the fast attack skills, other tactics might vary by build.

Dodge
You'll want to dodge instead of rolling or parrying. If you roll, you'll be too far away to strike them, and parrying you'll still take damage. Also, you will not be able to counter so holding down the Parry button is a waste.

Watch the monster carefully and continually circle around them. When they flash red, that is the visual signal that they are about to attack. Then you'll want to dodge to the side and get three fast strikes in. Then immediately back off. Don't get greedy, they can hit you through your own attacks.

If you have the Fleet Footed perk and Fast Attack perks, all the better.

Oils
If you have the Necrophage oil, and you've spotted the Alghoul far enough away that you're not in combat, apply your oil. A good technique for this is to use your Witcher senses to look for movement circles (the white wobbly circles) then listen, you'll hear gutteral gurgling and growling, which to me says Necrophage. Bear in mind you cannot apply oils when in combat, ie, Geralt has drawn a sword automatically and you can parry rather than Witcher sense.

Signs
You mentioned the Axii sign, indeed the Bestiary covers this. The sign is used when the Alghoul, after taking some damage, will produce large spikes from it's back. If you try and attack it in this state you'll take damage and will be staggered. It's in this scenario that you want to hit them with Axii, and wait a few seconds for the spikes to retract. Now you can continue to cicle, dodge and fast attack.

I also find some use of Igni to be helpful, but that's personal choice. Keep an eye on your stamina if using Axii and Igni.

Crossbow
If you are at a stage where you have acquired the crossbow, which it sounds like you have, you can apply some additional pressure with that if you're circling far enough away, or you do dodge roll. Just be sure to equip bolts with some silver damage otherwise it's a waste. Also be mindful of the often lengthy reload of the crossbow if the Alghoul closes on you quickly.

I used Quen to protect myself. It only absorbs 1 hit from them, but when you attack them it only takes away like 10% of your shield, so you can hit them multiple times without losing your shield or getting staggered.

Anytime you get hit by them though, you should dodge and wait until you can cast your shield again. I also was able to fight them only one at a time, after I drew one away from the other.