Cleric: really, act2 combat (prayer)?

[Draksen]

Hi there
Before D3 I started what will be probably my last D2 toon: a cleric.
I'm now level 23, and I have as suggested in many guides (and especially the one written by TheMadOnion) an act2 combat mercenary (using Prayer as aura).
the problem is that even if it's close to my own level, it's really squishy, and keeping it alive is a real challenge, even with a holy bolt spam against champions/uniques.
The other problem, which is in fact my main problem, is that it is BUMD. It moves slowly to the front line, when I try to put myself behind him he "strafes", avoid the mobs and I take the aggro. Sometimes it takes him 10 seconds to come to the fight, slowly, ... it's a nighmare, and I'm only in Normal. It will be unplayable in Nighmare/Hell, I'll die because of that :/
Is there a special tactic to use it, to make it more aggressive?
Or should I switch to another mercenary maybe? Act5 barbarian maybe?
Thanks for help and advices!

[Doctor Clock]

the problem is that even if it's close to my own level, it's really squishy, and keeping it alive is a real challenge

That seems unusual. Act 2 mercs can usually beat Normal and Nightmare for you with minimal gear. Have you given your merc a decent weapon and some form of life leech? You can try using Conversion if he is getting surrounded. From what I recall, the Conversion attack doesn't even need to land to convert an enemy so you won't need any AR. But really, life leech and feeding him the occasional potion should be enough.

The other problem, which is in fact my main problem, is that it is BUMD. It moves slowly to the front line, when I try to put myself behind him he "strafes", avoid the mobs and I take the aggro.

Could be one of two issues. 1.) Sometimes mercs bug and enemies just ignore them entirely. I think leaving and rejoining the game fixes it. 2.) More likely, merc AI is just terrible. Mind-boggilingly awful. There's really nothing you can do about it.

Is there a special tactic to use it, to make it more aggressive?

No. You can use Vigor to make him run faster, or get access to Teleport to quickly reposition him.