Sirius immediately recognizes that in his heavy armor it would be nearly impossible to even leave the tree-line unobserved. Also, the Talus and scree make riding a horse a risky proposition. There is a narrow footpath that winds its way up the slope (in full view of the guards) and a sufficiently skilled horseman might manage it, but Sirius doubts he could do it.
DC 15 Riding roll to negotiate the trail at a double move. DC 20 if you want the horse to actually run.
Donnybrook and Ery begin making their careful way toward the top of the slope. They soon realize that at some point they will have to reveal themselves and rush the sentries. They pause again about 30 feet from the top of the slope. The way from here forward is completely empty of cover.
Remember that 30 feet counts as 60 because of the slope...
Just as the scouts begin girding themselves for a charge, Grimble and Sirius spot one of the guards walking forward. He will almost certainly see the two scouts when he reaches the edge of the ledge. At that moment, also, Lucas completes his incantation and a tiny locus of his presence shimmers into being before him before fading into invisibility.
Casting is done. Grimble and Sirius have a chance to take 1 Standard action each before I roll for the bugbear's Spot check.