Challenge Routes are extra sections of Dig Sites in Fossil Fighters: Frontier. They are typically unlocked after the player has completed the story within that Dig Site. Within each challenge route is an obstacle course to be completed by the player using their Bone Buggy. The goal of completing the route is to reach the Rare Fossil at the end before the Fossil Eater, a Vivosaur that will damage the Rare Fossil until it is beyond revival, unless the player intervenes. Since Challenge Routes are meant to be completed in the quickest amount of time, there are only a few Bone Buggies recommended to complete the Challenge Route with the Fossil completely intact.
The Best:
Speed Demon: The Speed Demon is the fastest of all the Bone Buggies, with a maximum speed rating of ★★★★★. However, while it has more speed, it doesn't handle well on terrain other than road.
Turf Shredder: The Turf Shredder is the most oft-recommended Bone Buggy. It does have a ★★★★ speed rating, lower than that of the Speed Demon, but its ability to drive through all types of terrain with little slowdown offers the Turf Shredder more course options and better control.
Rock Crusher: The Rock Crusher, while sporting a speed rating of a mere ★★½, it has situational use in certain Challenge Routes, when the lightweight Speed Demon and Turf Shredder can't power through obstacles in a single blow, thereby actually increasing the amount of time the faster buggies spend in the route.
The Rest:
Light 4WD: The Light 4WD is the most basic Bone Buggy available, and is given to you at the start of the game. Unfortunately, this leaves the Light 4WD with far less desirable traits, including a mere ★★ maximum speed rating, equivalent to that of the Heavy Hauler, which has many more positive traits.
Digger's Rig: While the Digger's Rig is a reliable all-terrain buggy, it rates in speed at only ★★★, leaving it's only advantage over the Turf Shredder a slightly larger load capacity, which usually has no bearing on Challenge Routes.
Heavy Hauler: A very sturdy buggy, the Heavy Hauler only falls short of the Rock Crusher in speed, at only ★★. However, this is still tied with the aforementioned Light 4WD, leaving this to be desired over the more basic buggy.
Master Maw: While an AR card is necessary to obtain it, the Master Maw ties with the Rock Crusher in speed at ★★½, and surpasses it in load capacity. However, unlike the Rock Crusher or Heavy Hauler, it only has 2 sockets, rather than 4, leaving it to be overall less versatile in the field.
EV-405517: Nearly identical to the Digger's Rig at ★★★ of speed and the same load, but with only 2 sockets. It does have an interesting name though.
Goldrush Canyon
Challenge Route - Ilium Valley Road (★)
Ilium Valley Road is possibly the easiest of all Challenge Routes. The Fossil Eater is always placed behind a large boulder just off to the left of the starting point. In addition, the Route's Fossil Eaters stay asleep for a decent amount of time. Thus, either the Speed Demon, Turf Shredder, and Rock Crusher can easily complete the route with no problems, the Rock Crusher able to break through the boulder quickly, while the distance between the Fossil Eater and the Fossil is long enough for the other two buggies to make up for lost time with their speed.
Challenge Route - Boneland Way (★★)
The Boneland Way challenge route is a bit trickier. It features moving conveyor belts that can alter the course of a Bone Buggy. The area isn't too tricky, the key is to not get caught on the initial gate, or get swept off into a side path midway through the course. The Turf Shredder comes recommended over the Speed Demon due to its better overall handling, leaving less possiblility of being thrown off course.
Challenge Route - Marrowite Trail (★★★)
The Marrowite Challenge Route features four switches that need to be pressed to open up a path to a ramp that will launch the Bone Buggy towards the higher plateau where the Fossil Eater of the route resides. The Turf Shredder is once again recommended for its handling, being able to make sharp turns that will maximize the amount of time available to complete the route. Taking the rightmost path and then moving left one by one is the most recommended course of action, since the switch on the rightmost path is the hardest to hit, and taking too long on that switch is an early sign towards needing to restart the route. Be aware as unlike the last two routes this one features rouge Vivosaurs other than the Fossil Eater. Battling these Vivosaurs will not stop the clock on the Fossil Eater reaching the Fossil, so it's best to avoid them.
Challenge Route - Dinodome Path (★★★★)
Despite being a four star Challenge Route, Dinodome Path's actual difficulty varies between Fossil Eaters. Dilopho US is the easiest, spawning near the starting point and staying asleep far long enough for the player to quickly move in to battle it, leaving the only challenge moving upwards. Mapo is slightly harder, requiring some actual chasing. However, bypassing the boulder to the left of the starting point is quite easy, giving a small shortcut the player can use in order to ensure Mapo doesn't make it too far until it's battled. Dimetro is perhaps the hardest, residing up on the highest level near the Fossil, forcing the player to make their way all the way up the Dinodome, with various obstacles in their path. While Dinodome Path is mostly metal terrain, the Speed Demon still has trouble handling the sharp turns and subtle adjustments needed to complete the route as quickly as possible, leaving the Turf Shredder as an equally viable option for the route.