I am a level 34 engineer, and I found this guide very helpful. A few of my skill suggestions as to what I find useless in the long run.

Wither-IMO a waste. Didn't even bother getting it to 1. Just use leech instead of it
Empathy- max ASAP
Revive-keep at level 1 unless you plan on spamming it for the extra mp

Suppression-still have at level 1. Useful for if you grab some too many mobs. Also when you use it will auto-target (sometimes) the nearest enemy that isn't effected by the skill, unless all enemies were affected by it. However AoE skills like transference instantly cancel out the effect of the skill

Protection- I maxed it. According to what is said about armor I did some of my own testing and found 50armor reduces damage by 1, so thats what this skill will do, probably more profitable at higher levels as you will have more max health to benefit from this skill. And, it buffs the whole party too which makes me feel better. Again, optional.

Transference- USEFUL. Make sure to use it early because the healing wave comes in later than expected and sometimes you will die before you get the healing effect. It does about a quarter of damage to the enemies health bar, and heals you basically to full health if used on about four npcs.

Pain- Useful for taking out many things solo. It is very helpful on taking out the turret npcs, and in solo you likely won't take out npcs in three seconds like earlier said so the AoE stun at the end of the skill helps kill npc groups. Makes a great combo after suppression because the effect comes in right ad suppression ends. (Still at level 1)

Sonic boom- Didn't bother with this one. I don't think it causes any combos and the damage seems rather low for npcs. My hotbar is full enough already.

Decay- Not listed in this forum. I found the armor value decrease very helpful on select mobs like the vular tank npcs and other bosses. Dps is moderate too, I maxed it.

Please note that in this point of the game skill points come pretty easily. I am now at that point of the game where i'm just "maxing whatever." Currently everything in my hotbar setup that isn't maxed is leech and revive and suppression. And let me explain what my setup is:

First bar: (bottom to top) also known as the green bar, since all of the skills on it are green, healing type skills
Empathy
Transference
Leech
Revive

Second bar: kind of like my dps bar, and skills that stun or decrease the enemies lifespan
Pain
Decay
Suppression
Protection

Why is protection on my dps bar? Because its not on my healing bar (I liked the green thing too) It is a passive buff that lasts extremely long compared to other skills (two minutes) and I saw that it will be one of my least used skills so hence it is on the second set at the last of the hotkeys (number four hotkey if you ever played on a computer).

Another thing, my answer as to why engis are good at soloing. If you put it at pure dps (int) that is focusing on its main weakness and making it a balanced character. As for its low health, the healing powers and stunning skills compensate for that. As I said earlier, I estimated 50 armor decreases damage by 1 point (tested in UC marine corps on the second level, noticed an average damage increase of 3 when I equipped and un-equipped my 150armor holo shield) since one of the engis purposeful weakness is armor and we don't try to fix it (going strength) we can solo well. Also may this go to say that the str and int engi isn't as good IMO. I have actually had an engineer tell me my int engineer is better than his strength and int engineer (this forums reference

I hope my advanced guide will provide some help as to how engis do at higher levels (this forum is mainly built on level 20 engineer builds and is missing the second to last skill, decay). I would also appreciate it if you put my skill "green and mean" skill mapping guide right below yours