The role of a Med Kit Scrapper in both large and small scale fights is to support your allies, similar to other supportive roles such as the Firebrand. As such, your focus in the fights need to be on your allies rather than your enemies. That being said, don't forget where the enemy are or what they're doing, just focus more on helping your allies. Some of the support provided by the Scrapper is, similar to the Firebrand, cone-based. What this means is that certain skills affect targets in a cone in front of you, rather than in a big area around you. Notably, Med Kit #1, #2 and Elixir Gun #3 are of this type. Further similarities with the Firebrand is the Scrappers access to stability. They key difference however, is that most of the stability the Scrapper has access to is only personal. Every 8 seconds, you gain a stack of stability when evading an attack. Every gyro you summon gives you one stack of stability. The Bulwark Gyro toolbelt skill proveds 3 stacks of stability. Thus, there should be ways for a Scrapper to be where they want to be and use the skills they want to, at all times.
One way to look at the purpose of a Scrapper is to look at what it uniquely offers to the group. Simplifying it, there are three aspects that Scrapper does well and makes it worthwhile to have in a group.
1) Consistently high healing per second
2) High amount of condition cleanses (thus a lot of boon application due to the Purity of Purpose trait)
3) Great access to Superspeed
When playing, your goal is to maximize these three aspects in all ways possible. A key trait in the build is that the Purity of Purpose trait which makes each condition cleanse you provide, to yourself or an ally, convert that condition into its corresponding boon (https://wiki.guildwars2.com/wiki/Condition#Condition_conversion for a handy guide). Regarding the first aspect, your Med Kit provides great sustained healing for your allies with just the auto attack (#1). Just #1 doesn't allow your allies to sustain through a spike however, so make sure to weave in bursts of healing available through #2, #4 and the toolbelt skill (F1) between your #1 spam when necessary. It's worth noting that both the Med Kit #1 and #2 only apply to allies and not yourself.
The key to utilizing the Med Kit properly is to realize what each ability actually does and what your other options are to tackle any given situation. For example, Med Kit #3 is an AoE field that is put at your feet and pulses a few times cleansing conditions each pulse. Elixir Gun #3 also cleanses conditions, but it does so in a cone in front of you and cleanses one condition per pulse for five pulses. The important difference between these two skills, AoE type aside (cone vs field), is that Elixir Gun #3 doesn't apply to yourself. When your allies need condi cleanses but you don't, it's better to use the Elixir Gun #3 and save Med Kit #3 for when you need those cleanses yourself. If a specific person needs a burst of healing, it may not be necessary to use Med Kit #4 (unless you're low yourself), rather aim your Med Kit #2 towards them (an ally can be hit by multiple bandages, so the closer you are to the person the more you can heal them with #2). Similarly, if you only need to cleanse a couple of conditions for your allies and they also need healing, throwing out Elixir Gun #5 and possibly using F1 next to them, gives them the cleansing they need without wasting the better condi cleanses available through Elixir Gun #3 or Med Kit #3. Finally, to be a pro with the Med Kit you need to spam #1 as soon as you're not doing anything else more important.
Most of the condition cleanses have been taken care of already, but F5 is a skill that hasn't been mentioned yet. Provided you run the Supply Crate elite skill rather than the Sneak Gyro, the F5 skill is your best counter to an enemy condi bomb. It clears all conditions on the allies who are affected by the skill and thus provides every corresponding boon as well.
In short, condition cleanses are to be utilized in the following way:
Conditions on yourself and allies: Med Kit #3
Conditions on allies: Elixir Gun #3
Low number of conditions on yourself and/or allies: Elixir Gun #5
The third aspect of the Med Kit Scrapper is the regular Superspeed application it provides. In the provided build, there are three ways to give Superspeed to yourself and allies. Thanks to the Final Salvo trait, when a Gyro is destroyed it creates a lightning field that pulses 2 seconds of Superspeed over a 4 second duration. Additionally, the toolbelt skill provided by the Blast Gyro Tag gives 3 seconds of Superspeed in an area around you. You may want to use Sneak Gyro as your elite skill over the Supply Crate, but do so through an informed decision. The Sneak Gyro provides an extra Superspeed field as well as the possibility to support players who are far away from the commander and are under pressure through stealth. However, in large fights, more often than not, the benefits of the Sneak Gyro aren't able to be realized and it becomes a matter of Supply Crate condi cleanse and 2s 1200 range stun vs one extra Superspeed field.
UTILITY SKILLS
Med-Kit aside, there are a lot of useful skills that haven't been discussed yet. In this section the notable skills will be listed along with typical use-cases.
Hammer:
You shouldn't be in hammer for 99 % of the fights, but there are still great uses for it. Firstly, Thunderclap (#5) should be used to stun enemy targets that have no stability to help the group kill them. The most typical situation is when a warrior of ours puts down a bubble stripping the boons of all enemies inside it.
Skills #2, #3 and #4 on hammer are all situational and can be used to save yourself or others in various ways. #2 reflects projectiles and can be used while you're trying to get out of danger or to body block for an ally being targeted by projectiles. Same goes for #4, which instead of reflecting projectiles, blocks attacks for 2 seconds. The leap (#3) can be used to get away from enemies while in combat.
Elixir Gun:
The #1 and #2 on Elixir Gun are fairly useless and #3 and #5 have already been discussed. However, #4 is a great positioning tool to master. It's a 550 range backwards leap, as well as a blast finisher. To best utilize this skill, you should bind "About Face" in the control options to an easily accessible key. It allows you to quickly move out of danger or to reach an ally in need quicker.
Gyros:
The Bulwark Gyro is quite useful. Despite almost always being destroyed immediately, the 33 % damage reduction buff it provides lasts for 4 seconds. Use this when your group is under pressure.
The Blast Gyro Tag is honestly pretty bad on its own and is taken for its low cooldown (= more Superspeed fields) and its toolbelt skill. Typically, you manually destroy the Gyro (by using the skill again) as soon as you spawn it to provide the Superspeed.
Elite skill:
The Supply Crate is quite bad as a skill due to its long cooldown and low damage (due to your minstrel gear). The only use for it is for a long stun on targets without stability. As such, the key moment to use it is in a good warrior bubble.
Toolbelt skills:
F1 is a big heal, both for personal and supportive use. Remember that the cooldown on this resets if you hit 25 % health.
F2, provides AoE Superspeed as well as a personal stun break.
F3, gives 3 stacks of long-lasting stability to yourself and allies as well as creating a projectile-absorbing dome around yourself which is great for countering projectiles in fights or covering siege from siege disablers.
F4, personal stun break. Save this for when you really need it, it's your only reliable stun break. While F2 provides a stun break too, you should be using that to help your allies more than yourself.
F5, as stated before, is an amazing condi clear. Use it when condi-bombed.
As a final note, the Function Gyro mechanic can both help you out a ton in battle and have virtually no impact. Stomping isn't really a thing in most large-scale fights so avoid summoning it on downed enemies as it'll then be on a 20 second cooldown. However, if you summon it on allies who are downed and need to be ressed you will speed up the ressing considerably. Besides, if the Function Gyro completes its action (whether stomping or ressing) it will proc the Decisive Renown trait and thus provide a 3000+ barrier as well as 4 seconds of Superspeed to allies. This trait is procced when you res/stomp by yourself too, though.