Spire is the only ship I fly and I've studied it for a while. It's not as versatile as the Mobula at the moment and is much more situational. I'll start off with maps since they usually determine if you should even consider the Spire.
MapsCanyon Ambush: Best (Fog, narrow paths, sufficient cover)
Paritan: Good (Narrow paths, sufficient cover)
Northern Fjords: Good (Fog, fog, more fog)
Duel at Dawn: Average (Too many flanks, but small map)
KOTH: Average (Spire is good at brawling, but too fragile to hold the point)
Fight over Firnfeld: Bad
Water Hazard: Bad
Crazy King: Bad
Dunes & Sepulcher: Terribad (Dust cloud makes it hard to approach, and a blind person can see a Spire coming)
Like Fynx says, Spire is a poor choice on long range maps. Its optimal range is close to mid. It will never beat Mobula in a head-on, long range engagement. Spire is a large target, while Mobulas are much harder to hit. Lumberjack is also out-classed in range by Artemi and Mercs due to their high projectile speeds, hit-box and ease to repair.
MindsetThe Spire is a gunship - if you're repairing most of the time, you're doing it wrong (or your allies are). Getting a good initiation is important. You want to stay hidden and charge in with all guns blazing at the right time. When the chance presents itself, get in fast, take down a ship, and finish the engagement fast. If the Spire gets hit, there's a good chance it will die.
I'm happy Spire pilot whenever I have a Galleon ally poking the enemy and drawing fire. Or a Squid that scouts the enemy position while I stay hidden, looking for an opportunity. But almost all the time I get Junkers and Goldfish that stay behind me, expecting a Spire to take point.
Crew managementA lot of engineers are used to repairing - making everything scratch-less and spotless is their priority. This is not how a Spire should be crewed. Only call for repairs when there is little chance of fighting back, which happens often when you get hit by disable weapons.
I run both engineers top deck (all the time). The main advantage of this is that the Gunner gets a second gun to shoot (note: it's the ONLY ship with two guns this close). Back when stamina wasn't implemented, a Hwacha reload is 11 seconds for your Gunner to not do anything. A second Gatling firing would help break armor for the Mortar and second Hwacha volley. With novices I'd still teach them to run top deck - some perform well, some don't. With that said, it is up to the pilot. I have seen many pilots and engineers saying one is better than the other. Practice and discover what works for you.
Tryhard stuff here. If you have a good buff engi, put him on Side Gun, Armor, and Main Engine; keep them buffed when possible. A buffed Main Engine helps a lot in your approach or kiting, and Armor will give you a better chance at surviving. Also, bring the ammo type for the forward-facing light gun, just in case your engineer is busy repairing and misses hull breaks or important Merc shots.
BuildsFirstly, these are the builds I use, and there are many variations that other players use. So, you can have your own builds, and I totally respect that.
Hwacha Spire1. Hwacha 2. Mortar 3. Gatling 4. Gatling
It is easiest to see the Hwacha Spire as a combination of a Hwachafish + Metamidion. Hwacha disables the opponent, and Gat Mortar quickly takes them down. Remember the Gunner should be shooting the Gatling while the Hwacha reloads (unless the situation calls for him to stamina-reload).
Lumber Spire1. Lumberjack 2. Merc/Hades 3. Flare Gun 4. Banshee
Good build for Fjords; lots of mid-range killing power. One of the best at quickly taking down Galleons. This is what I would call the meta-Spire or the signature Spire build because reasons.
No Minotaur please - the Hwacha is much better at everything, besides nudging ships. Something similar can be said about the Heavy Carronade. It is good against Mobulas in close range maps, but so is a Hwacha.
Beating a SpireThere are ships that are weak to disables, and there are ships weaker to straight up kills. Spire is weak to
disables. Getting a full Hwacha volley on a Spire means they're out of the fight for a good amount of time, while being completely vulnerable to a quick kill. Artemis and Mercury are also strong weapons to wear down a Spire. Attempting to kill them without disabling usually means receiving a murderous amount of firepower.
To end this off I would like to say...
damn you Mobula.