Engineer newbie... need tons of help
honeslty, everything you mentioned worked. Your best bet, is to play with the different kits, gadgets, elixirs, rifle, pistol+pistol, and pistol+shield, and decide what feels like it plays fun for you. Because there is absolutely a viable build for just about any combination.
Since you specifically mention dungeons though, as stacking in a corner is a general system locked in the minds of the masses, I find bombs ideal for this. Otherwise, try some things out and decide what feels fun. Once you do that, we can easiliy tell you how to build for it.
https://www.youtube.com/watch?v=6q3em9s5I4c
The main strength of an Engineer is that with Grenade Kit/Bomb Kit he can inflict 20+ stacks of Vulnerability and generate 9-15 stacks of Might. If your team doesn’t need the 20+ stacks of Vulnerability (e.g. because there is already a Blind Exposure Guardian and two 30/25 Axe/Mace Warriors in your party) you mainly use Bomb Kit #1 and only use Grenade Kit for #2-#5, or maybe even replace it with a different utility skill.
Leveling tips:
https://forum-en.gw2archive.eu/forum/professions/engineer/Updated-Engineer-Leveling-Guide/first#post2970327
More general information:
https://forum-en.gw2archive.eu/forum/professions/engineer/Engineer-PvE-little-help/first#post2992442
Dungeon usefulness list:
https://forum-en.gw2archive.eu/forum/professions/engineer/Dungeon-usefulness-list/first#post3114211