The baddie AI tends to target the henchman healer more often, particuarly in the areas around 'the wall' - but it's not ALWAYS..many times the monsters split up and target Alesia, Orion, and myself all at once. There was one pops-out-of-the-ground monster mob type near Hedge of D. that almost always went for me (E/Mes) instead of Aleisa or anyone else, and she did great at healing me while I cast Lava Font till they all died.

Course, it only takes one good horse archer or something to keep Alesia occupied w/healing herself instead of anyone else... lol
However, I also have had times where she just stands behind me, initially - or any of them, really - until I've actually got off a round of attack first (ie, my first spell completes casting). Sometimes if you press the spacebar instead of just 'closest target'+'attack skill button' it seems to trigger henchies to attack, if they're doing the 'stand and pick their noses' routine.
So to Kosiam - you don't say how far you've gotten in the game...but if it isn't too far, I'd say...try to stick it out till the Wilds, you may find yourself adapting to their quirks and they'll get stronger/better skills and be more helpful, too. Alesia hardly ever dies anymore, for me...if she does, it's because I got over-confident/wasn't cautious enough and I would've been trounced even w/the best AI healer ever. heheh
Edit: sometimes word filters filter out not-bad words along w/the bad. sigh.