If a hired mercenary or summon kills soething, do you still get the experience?

Yes

But hired mercenaries suck (they die too soon) against enchanted creatures. If you want summin' done right you gotta do it yourself ;)

I completely disagree. With the expansion installed you can give your mercenaries items. They also have a ridiculously high "main" stat. They also get horrendous resistances. Many a time have I sat back and let my mercenary kill Mephisto :D .

can any one give me tips on how to kill Mephisto, nice and easy? I always have trouble with him, even with my lvl 26 barbarian....i think i may not be using the right tactic...anyone know what to do?

The easist method is to buy a bow and do the moat trick. ie, go to mephisto and run back as fast as possible, mephisto will be block at the moat and do nothing. you can then shoot him with ease.

For Normal-Nightmare Mephisto. 5-10% Life Leech and some decent Cold/Lightning Resistances will enable your Barbarian to smack Mephisto face-to-face. If you have an Act2 Merc with some Life Leech, he can help you whittle down Mephisto's life too.

For Hell Mephisto, melee is still viable, but more risky. If you specialised in Leap Attack, now is the time to use it. Otherwise melee with more Life Leech (Hell Difficulty imposes a penalty) and good Cold/Lightning Resistances.

so basically, u need life leach to dealwith maphisto (or at least make it easier) . Are there any otehr things that can help?

If you have install the new 1.10 patch. You can't leech from mephisto or any other end boss monsters anymore.

The bad thing is it that I was playing Diablo II (I'm a level 25 paladin now in Act II) and I have done the Canyon of the Magi and the (good) Tal Rasha's tomb some dozen times already only to find the game crashing at the time when I put the Horadric Staff in the Orifice. Really pisses me off as I want to kick the living hell out of Duriel.