I am here for only a few months but I can share a bit what I realized here, coming from official classic 2 years ago.
Any melee class have no problems to level up from 1-76, main + WC/PP + BD can do efficiently enough.
Any mage class can do the same but less efficient as no vampire and mp could be a problem. Furthermore, melee can use macro next target>attack easy, but mage need to manual click on target as some skills ranged 900 which next target cannot be used or you dont really want to get close to mobs.
1-70 is easy, up to about 50, RBs daily can easy bring you up to 70, actually 76 but slowly later on
form 70, if you want to solo LOA, you need to invest some gears and try to optimize your solo party with multi box
what I mean is taken (hex, stigma, gears) efficiency, (heal, mp) sustainability into consideration
Ideally you want hex and stigma to allow you farm efficiently without costing much on gears, I believe either one debuff here have better effectiveness compared to a QA3 which cost you over 3bs (didnt look deep on maths here, correct me if im wrong), so consider cost effectiveness on gears and party setup to get a good balance to achieve what you want
Im still 70s, trying to touch GC but that is totally different levels
Using B gears to tank giant is just so crazy, A gears is getting common but +6 set cost you at least 2-3b
If you want to solo multi box in GC, you really need to plan early. From my test, you need Main + WC/PP( ideally both for cov and cc), BD+SWS, SE with some good gears but that can be discussed later when you are 76