There are three steps I go through to decide which build I want to use.

Step 1)
Look at the enemy team and decide weather they are squishy or not. If the enemy team is fairly squishy then would go with the full offensive build so I can hit and run. If I see the enemy team can take lots of damage then I would go with the Semi defensive build.

Step 2)
Look at the enemy and your team's builds. This part can be a little bit tricky because you should know that you can morph the builds together. If the enemy is going to get defensive builds and are not very squishy I like to use the Semi Defensive build and switch out Worm Hole for Assassin's Blade, then possibly Stoneskin for either Berserk or Double Bombs. If the enemy team is squishy but going for a bit of a defensive build then I will go for an the full offensive build and switch out Seasoned Marksman for Regeneration Master and Berserk for Stoneskin. If your team has lots of stuns going for it then I will run with Shadow Assault but this is very rare because of how powerful Void Prison is.

Step 3)
Is your team bad? If you are playing hero league or quick matches then it is important to look at how your team preforms. You have to decide weather you may have to go with a defensive build because you don't have a support or your team all together isn't surviving much. If your team isn't surviving for a long time and not getting takedowns I recommend switching out Gathering Power for Focused Attack.

Tip: These builds are not set in stone and you should always decide what you use depending on the other players. Morphs between builds are very helpful and can decide a win or lose.

Never use Blink offensively unless you have Wormhole because if you Blink in to try and get a kill chances are that you will most likely die so it is crucial that you save Blink to escape.

Team Fights
If you have Void Prison what you want to do with it is try and swing the fight in your teams favor by trapping 2 enemy opponents in it. Your team can then capitalize and sweep up the three or so that are not trapped and then pick off the others.
When you attack you usually want to open with Cleave auto hit then Singularity Spike auto hit then try to re-stealth if you are getting a little bit low on health. You always want to auto hit between abilities so that you can take advantage of your follow through talent.

Roaming
What you will have to do is find anyone that is vulnerable. Never get hit by towers because that could mean your death or get you really close to it. You will open the same as if you were in a team fight.

Contact me if you have any suggestions or changes you think would help.