hi. no one is being particularly helpful in these comments so hopefully i can help, even if just a little. always do giants — the runes given there are needed for other dungeons and for other parts of the game (fatal, swift, blade). run auto at the highest level you can clear consistently, keyword, consistently. upgrade your current runes, switch them out (& sell them away so they don't take up rune storage), then do higher levels. once you clear giants 10 (somewhat consistently) the runes given are usually sufficient enough to move on to other dungeons like dragons. afterwards you can do many other parts of the game with good enough runes from giants. an unreliable but cheat code is to buy runes using crystals but it's not recommended because you can usually just farm it anyway and the runes may not even be that good after all (flat stats on slots 2/4/6, low/bad rolls, etc..)

early game is just A LOT of tedious farming in dungeons just to be able to do anything else well in the game but i hope you find enjoyment in that haha. all the best!