Omega is a COM with Late Bloomed (i.e. max level 99), Sparkprone, and Heartless.
Passives Abilities: Feral Surge, Feral Fatigue and Leadenstrike.
Starting Attributes:
Attack 595
Magic 479
ATB Level 3
HP 3241
It doesn't seem like there are any mega bosses that pose a serious challenge for fully maxed-out players. Raspatil's lack of Doom/Wound damage can easily be exploited by having a Sentinel out, and his strongest attack can be blocked with the Tortoise paradigm (SEN/SEN/SEN), allowing even under-leveled players to emerge victorious with a fairly simple strategy. Fully-leveled players will be able to achieve a five star rank on this battle (among other supposed "mega bosses") without needing to employ unique strategies like those that were needed to defeat/five star the Long Gui, Vercingetorix, Neochu, and Gigantaur in FF13 (the four hardest bosses in my opinion, and all harder than any boss that exists in 13-2).
Ironically despite that the advertisements for the game talked about how Wound damage would increase the level of strategy needed to beat the bosses and stronger enemies (so you couldn't just spam Sentinels/Medics), the game's battles have less of a sense of urgency than in 13, where many of the bosses casted DOOM.
Sadly the grading system in 13-2 rewards players for having a higher level instead of taking the enemy down with the most effective strategy. In 13 target times were adjusted according to your stats, which meant it was possible to five-star an enemy if you were severely under-leveled as long as you were strategic in your approach. In 13-2 the target time always remains the same - so essentially the game prizes players who spend more TIME playing the game instead of players who IMPROVE their skill.
The most strategy-inducing battle in 13-2 is probably Yomi, who unfortunately is just an easier version of Vercingetorix from 13. Against Vercingetorix, even fully-maxed out players can have difficulty, especially if going for a five-star ranking. The poison strategy requires actual manual ability selection, a quick-fingered switch to SEN/SEN/SEN to reduce damage from Wicked Whirl, and a good setup beforehand. If you want mega bosses, play 13 - the Neochu, Gigantaur, Vercingetorix, and Long Gui fights will be much more satisfying than any battles in 13-2 when looking for a challenge.
Best Answer
There are in fact, a finite number of artefacts in the game. These are split into two categories:
Wild artefacts- There are only ten of these in game. You can only use them on gates that don't need a specific artefact. If you are stuck finding these, there's a handy guide hereRegular artefacts- Each of these correspond to a specific gate and are generally to do with story progression. As you'd expect there is one for each gate. Likewise, you can find a guide to finding these here. I wouldn't recommend using this guide though, whereas the wild artefacts can be difficult to find, these are part of the natural story progression and not that difficult to obtain. They're generally awarded as a sort of 'Level Complete' rewardOn another note, it is strange that your brother of a friend should restart the game. You can complete the game without locating all of the Wild Artefacts. You will only need them all to collect all 160 fragments, and they don't need to be collected and/or open gates in a specific order.