I use regen and medica II all the times. They are pretty much the core of healing from my point of view (people may disagree but that's not the topic). I never had an enmity problem with these (except Garuda' sister, I had to adapt).

So, using regen and medica shouldn't be a problem. The tank might be doing something wrong, or you might be doing something wrong.
Change the tank, try another one, two other ones etc etc... if the enmity problem vanishes, then it was the tank doing something wrong.
If the enmity problem goes on, it might be you doing something wrong.


However, titan have another specific mechanic: the rest of your party may suck.
If you are healing dps after each plumes, after bombs, after all these damages your party should avoid and still take, then the problem is there: they should dodge, not rely on you to heal them for everything.