These guys are pretty slow by themselves, have a poor turning radius unless in lava and even puff up their gullets like an angry frog if you get too close. Just shoot them in the back from behind if you need to kill one. To the get eggs however will take a whole team. Note that there are bats, centis, scorpions and other nasties in the cave. And like 20+ magmasaurs guarding the nests. Just bring a few friends, some bog spiders to distract the horde, pick them off, and bam. You got magma bois. A fun little detail I've noticed is that they seem to be at maximum power when their bodies have an iron, blueish colored tint to their bodies. Beware of this, as their lava burst attack is at it's strongest in this state.
If you get too close to a wild one that used it's special ability (a charge that will set it on fire and aggravate nearby dinos) it'll display a message saying "getting hot! Stay away!!!!" and that will set you on fire. Don't approach unless you know you're prepared.