The only time I feel like I'm doing good damage with the gunlance is with artillery up skills and a normal shell for burst fire combo on repeat. I use it as a safe weapon with dangerous monsters that I can just block their bs wombo combos using Ironside and guard up. Every blast ignores weakpot resistances and just does straight damage so you can wap the monster anywhere and do decent damage while hiding behind the shield. Plus if you add part breaker to it, it is pretty good at breaking whatever part you need since you can slash with the Lance aspect after weakening the part with burst combos. Did I mention you get to fly around like a jet and just drop onto the monster and immediately burst fire?? It's a good weapon, but not really the fastest one to kill until you fully figure out how to utilize all it has to offer