Defeat Baron in Mar's tomb was a mission in Jak II. After passing the First test of Manhood and the Second Test of Manhood Jak finally gained entrance to the room holding the Precursor Stone. However, before he could take the Stone Baron Praxis interrupted him and tried to take it for himself.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Grin and Baron It", in which after completing this mission you will be rewarded with a silver trophy.
This time it is Praxis in his Widow mech, though for the first phase he'll be busy trying to rip the Stone out of its holds. Much like last time the mech has three health bars total, resulting in three phases.
The Baron will launch several spydroids onto the stone ring which is your only place to walk. They can't be destroyed using melee hits so instead use the Scatter Gun with its wide cone of fire to fend them off with ease. After this the Baron will release several bombs (that release firework of sorts when they're about to explode). Make sure you line yourself up with the bomb and the Baron and kick it back at him. Do this four times to start the second phase.
This time it's nearly the same as phase 1, however after he launches the spydroids he'll jump down from the statue and periodically fly up in front of the stone ring and launch a couple of shots while moving sideways. It is not possible to dodge normally so instead stay behind one of the stone pillars (be warned they can be destroyed by his bullet fire) and take care of the spydroids as they still will be coming for you. After this repeat the bit with the bombs, only to have the Baron actually manage to take the stone for himself.
With the stone's power he'll fire a strong attack that will destroy part of the stone ring you're standing on. He only aims for the sections between the pillars though so use this knowledge to avoid his attacks. You still want to be hiding behind a pillar as he'll move sideways and fire a couple of shots (slowly destroying the pillars as well). In between this he will have launched two bombs in a random direction. Use the short pause after he fires his bullets and moves elsewhere to go to a bomb and kick it at the Baron's mech.
After taking off 50% of the last health bar he'll launch the bombs before he fires his Precursor Stone attack, technically making it easier if you manage to kick them in time. If you won't however he'll most likely hit you when you go for the bomb with his bullet barrage. And even if you finally manage to defeat him, he'll still fly off with the Precursor Stone!