How do you fight against Geovishap Hatchlings?

Intended way (it's even mentioned in advice piece your are receiving after meeting them for the first time): wait for them to drop crystals, collect them to receive a shield buff, tank their attacks while protected by shield.

While trying to record the optimal strategy I figured out they moveset:

Melee attacks:

• regular slashing attacks with one or both arms;
• spinning in place attack.

Ranged attacks:

• rolling around full Sonic that mentioned in this question;
• jumping onto you with traditional red circle on the ground;
• not really an attack - but strategical burrowing with nice side effect of dropping geo crystals. For me this one most often happens right after jumping attack if you run away.

So the best strategy is to stay away from them - they cannot use any melee attacks and are limited to ranged ones. Wait for jumping + burrowing combo and run to collect crystals. Crystals will give you geo type shield and you will be able to tank any of their attacks for some time. This is time to run towards them disregarding what they are doing. If they do sonic rolling attack and run into you - they will get stunned and be laying on the ground for some time.

My initial advice about elemental reaction was only partially correct: they are indeed can be stopped (without stunned animation), not by elemental reaction, but by dealing enough damage to them. Could be both melee or ranged attacks.