I guess you're doing this to get enough Orbs to proceed past the blocked door, right ? There's actually 45 Orbs spread around the room if this is too much hassle for you, just in case you didn't know that already.

Getting onto the spear is a tricky one. It's been a little while since I've done it, but if I remember it right the easiest way I've found is to walk against it while on the horse's back so you're right up against it and facing it as directly as possible. Stand still at this point, then jump directly up, and while in the air, not while jumping but after you leave the ground, hold the direction towards the spear. If you do it right Dante should jump directly up, then do the ledge-grabbing animation as he begins to fall and lift himself up onto the top of the spear. Do a second jump directly up and it should rain Red Orbs for you .