For a engineer in elite hardcore i would surely have my drone to be a melee drone to start with (avoid a rifle drone, their lazy tanks and will get you in trouble).
And i would max rocket bots together with molotov assault (great dps and fun to use).
If your going to team a lot i would strongly suggest taking haste bot aswell though. Or just use haste + rocket bots as your 2 active bots.
And beacon is something you dont wanne overlook aswell together with tactical stance (although you wont have spare points to have tactical stance more then 1 and beacon more then 5)
And what exactly did you link that get warned for? For posting a outside link for a class guide or a class tree you should not get warned, perhaps the post content?
Can link a skill tree for a engineer i have in mind for hce play later if you want , but for now im done for today gnite