Usually the best mercenary is one from act 2. The are good tanks that can take a lot of heat and deal out some good damage. On top of that they have auras like Holy Freeze and Might that can really help you a lot. Some builds choose an act 3 mercenary because they lack elemental damage, but even though it helps act 3 mercs do lowish damage. Act 5 mercs are probably a bit better at tanking but lack the auras that act 2 mercs have. Act 1 mercs can also deal some good damage and are good at staying away from danger.
Assasins aren't usually the most powerfull of characters who can just tank everything, so my suggestion is an act 2 merc with either Holy Freeze/Defiance/Might depending on your needs.
Also it it usually a good idea to focus your points in one skill tree...The assasin is an exception since she can benefit from points in all 3 different skill trees. If your main focus is on martial arts you should probably concentrate on getting only Death Sentry in the traps skilltree since the other traps won't be nearly as useful without synergies...and if you're mainly martial arts you won't have enough points to max synergies.
If you're interested in a more thorough explanation of mercenaries there's an excellent guide in the Single Player Forum...
But do might increase elemental damage as well or is it just physical? Thankyou very much!
It's only the physical portion of your damage..The enhanced damage from might will come of your weapon damage. You're welcome
You don't have to be a specific lvl to get a mercenary that is near your lvl. Unless you are really high lvl the mercenary will be almost the same lvl as you. I find that it is a very good idea to be at least lvl 30 before entering Nightmare. Normal Baal runs is a very good way to level up and you can quite easily get to lvl 40-45 and then enter Nightmare, that would also make it much easier for you.
I wonder some more things.
I´ve put one point in Death Sentry so far, how many points do you think I shall put in it?
And shall I max out Shadow Master? I thought that having a good shadow master and a mercenary with thorns could be quite good.
Is it? Or do you still think that it wouldn´t be a very good idea?
I want you to know that I really appreciate your help! Thankyou very much!