Upgrade parts that support a coherent Campaign build and fix a measured damage or survival shortfall. Attachment levels add 3% HP and damage per level; ranked Arena normalizes those levels, so upgrades serve a different purpose there.
Find the part doing useful work
A weapon that consistently reaches and hits the target is a stronger upgrade candidate than one that spends the fight chasing or pointing away. Likewise, a defensive module worth keeping alive deserves attention when it is the part whose loss triggers defeat.
Do not assume weapon damage is always the first investment. If mobility breaks before your saw can work, preserving movement may unlock more damage than another saw level. Use the replay to identify the decisive failure.
Understand the level rules
Each non-core attachment level adds 3% to HP and base damage above level one. The multiplier does not increase weapon range, reduce cooldown, remove heat costs or improve core body health.
The part-level cap is the ceiling of Inventor level divided by three, plus four. If the upgrade is capped, progression is the route forward. Check the Workshop’s current quote and cap rather than using old fixed maximum-level advice.
Keep Campaign and Arena goals distinct
Campaign retains attachment progression, while ranked Arena strips attachment level advantages. You still retain your owned mechanics, loadout and Brain choices in Arena. A higher-level part therefore does not carry the same advantage into that mode.
Start by investing in a small set of useful pieces, then broaden when a new matchup calls for another role. Use available missions, rewards and reached Campaign fights to support the plan. Avoid spreading Scrap thinly across equipment that does not yet belong to a working build.