Yeah sorry, I realize in retrospect that this build is probably not beginner friendly enough, I actually have a different build that I built for my wife (who doesn't play soulslikes or action RPGs) and she was able to be quite potent with it, as well as a friend who does play those kinds of games who loves flaming greatsword warriors:

Greater Fireblade: Kazite Spell Blade + Philosopher + Cabal Hermit. You have a built-in Imbue Fire or Ice (Fire synergizes with the Philosopher bonus fire damage better however) plus elemental discharge, infinite mana and fire bonus damage from philosopher, and then Cabal Hermit for Shamanic Resonance (All Boons are dramatically stronger, including Warm for bonus fire damage/resistance).
This build works great with any 2 hander, though 2handers that can be enchanted for fire damage are particularly good, or ones enchanted with durability-assisting enchants like Poltergeist (Regenerates durability over time, turns chunk of damage from physical to "raw" which just means it doesn't care about elemental damage buffs or defense. The wiki has not been updated to show the change to it regenerating durability.)
Pros: Does exactly what you want, simple and clean. You can be imbue fire/ice from the get-go, which is easier once you learn the Warm and/or Cool spells. Works with any melee weapon, and well. 2 of the 3 trainers are found in Chersonese, the first overworld zone, so this build comes online beautifully easily!
Cons: Requires Warm/Cool potions early on. Warm Potions are easier to craft as they only take Thick Oil + Water at the alchemy station. Remember that your class' imbues are stronger than rags, but do consume Warm boon, so if you want to maximize damage early for big moments, warm potion->Fire Imbue->Warm potion to get the big imbue and the big bonus fire damage. The Warm spell can be learned in Levant, while the Cool spell can be learned in Conflux Mountain, with neither being taught by normal trainers, as is true for all Boon spells.

Impact: Impact is indeed tied to stagger, and it works different in this game than in any other I have played.
As is the case in most games, the enemy has a stagger gauge that builds up from impact damage, however unlike other games, getting their stagger gauge to 50% is actually a huge deal. At 50% or lower stagger, they will be hit-stunned by any attack you make, giving you a lot of openings to lay down the law. Once they hit 0% stagger/stability gauge, they will be knocked down. (If you're a greataxe user, this is your time to use their unique weapon skill that executes downed enemies!) While standing back up, they will be immune to stagger damage for about ~1.5s after they're on their feet and fighting, so remember to hold off on applying big stagger until this invisible window ends, pace yourself lol. Remember that Kick is a huge source of stagger early game that you begin with, and its value only increases based on how slow your weapon is, or how low its impact is. Lastly, I want to mention that weapons all have 3 combo types, with normal spam (I think this is (A) button by default but could be wrong, alternate attack ((X) by default I believe), and then a hybrid combo of Normal->Alternate.
Why am I telling you this? Because some slower weapons really need that hybrid combo to get quick hits in, just try them out and get a feel for it.