repeater often over-kills mobs because the 1st or 2nd bolt may kill it but all 3 will still fire.
This is about the only practical use I've found for IPS these days. Hunt's End applies to each of the 3x shots. So, first bolt kills one, second kills another, 3rd kills another. With PS, 2nd & 3rd bolt try to kill the same dead target over and over again.
Does this not mean something is wrong with Mechanic and Gxbow, if we have to multiclass 4 levels of arti
It's a value judgment. I've never played a pure class or wanted to play one. I view multiclassing as part of making a character do what I want rather than what the devs decided. Worst case, I get irked when I have to "shop around" too much; eg. good Trance and Boost are often missing from old trees.
For example, I love the 3 Swash splash. I don't think Bard or Swash are broken because it's rarely taken past T3. Instead I'm glad it only needs 11 AP and 3 Levels.
Mechanic, however, is a bad splash and a bad tree. It's cores are mostly pointless; only C5 is useful and it likely belongs down around C2 or C3. Most of the tree is junk. The unique attack line is contruct-only and thus worthless. The trap features rely on lobbing physics, which badly need fixing for speed, reliability, range, and fluid play -- it's 30 AP wasted to unlock the useful nodes and then the T5 fails to deliver. After all that AP, your character doesn't perform as well as a repeater, INQ, or shuriken ranged.
Despite GXbow badly needing help, I'd still fix it last of "the things that need fixing": casters and lobbing physics (which could make alchemists MUCH more fun and help GXbow), then rangers (aa+tempest), last gxbow. Tempest & Gxbow last because despite not competing, there are still v.good twf & xbow builds.
It would be nice if every tree worked both for splashing and as a full tree; Kensei is an example of that. Mechanic is one of the sad lonely trees, unwanted as either splash or full.