Blast Cannon -- 1 point early (2 if the defense debuff still stacks despite that supposedly being patched), then stop at 5 until you can afford increasing mana costs, and eventually max Heavy Lifting -- Keep as high as possible at all times Fire and Spark - max and use a weapon with these elements whenever possible Coup De Grace -- I've heard varying opinions on how far to take this, though it's more important for cannon engineers. You need at least one point for sure. Immobilization Copter - max Sledgebot - max
You have some freedom from there. I've heard recommendations for Spider Mines (for kiting, hitting stuff around terrain without exposing yourself), Fusilade (tons of area DPS after level 10 but completely useless on anything that charges you), Aegis of Fate, Force Field (with 1 Charge Domination), and even Gun Bot.
"That is why war is so tragic. To win means to make victims of your opponents and give birth to hatred." - Kratos Aurion, Tales of Symphonia
Definitely max Healbot and Forcefield, but don't put anything in Aegis.
Coup de Grace is a strong one-pointer, though it may be bugged at the moment.
Fire and Spark ends up being a waste because there are no good cannons to use with it at high levels.
Vit is practically worthless to anyone not using a shield (and even with a shield it's still of dubious value). You should have at least some dex though; I went with 110 dex and everything else in str for my cannon engineer on elite.
Max the following... //Aegis// - Forcefield - Charge Domination
//Construction// - Heal Bot - Gun Bot - Sledge Bot - Blast Cannon - Fire and Spark
//Blitz// - Heavy Lifting
You are going to end up with 12 spare points to distribute as you see fit. In the harder difficulties such as Elite, Forcefield will be the skill to consume your charges. Forcefield grants you 50% per charge, IIC. It will be especially invaluable in NG+ and NG++ modes.
Heal Bot will always be of exceptional use, but Gun and Sledge require a full investment to get the best possible return from spending points on those skills. Fusilade is virtually worthless right now, so wait until someone fixes it before you so much as think about investing in it. But once someone fixes it's targetting, it'll be a boss crusher for sure.
Maybe if we talk to the television, and know everything there is to know about football, that our favorite team will make it to the super bowl...
Forget Cannon Engineer even exists, go with an Outlander or Embermage if you want ranged. Cannon Engi isn't even close to being as good as either of the above, but they play almost exactly the same way, just takes longer to kill stuff.