Do I have to trigger a heat enchant?
You need to get 100 focus by successfully attacking / blocking / parrying and then press Right Bumper on your controller and then choose a rune spell that is assigned to weapon in your right hand. You can also get a Channel rune spell, which will give you 50 focus after a bit of charging time - you lose hp continuously and then stop the cast and get 50 focus instantly. You can also drink potions to get focus.
So the spell isnt on a timer, and doesnt “ready”, I just need enuff focus? Thanks! Are there weapons that do elemental damage without hitting extra buttons?
Hm… You can get magic items that deal heat / frost / lightning / plague damage on top of the physical damage and it is possible to make a build where you only cast spells and regenerate enough focus to be able to cast all the time (although this may be nerfed in the future).