I wouldn’t say it is undoable (as there are players who claimed they’ve done it before), but it will be quite a challenge if you use 3^50 3* x 4 and only 3^60 x 4*. I am not the best player by any stretch, but I couldn’t finish the last stage on these Rare Quests on 3^60 x 3 4* and 3^50 x 2 for a few months.
The general rule is to survive the first mob from fresh, you may need to mana up a few of them using mana vials (small one, that you can bring 10 in), survive the first while building up your specials, you might need to bring 2 healers with you, but that will generally means cutting down on the specials damage output if that’s what you rely on. In the mob wave before the boss is where you leave one enemy alive and start building up your specials by ghosting tiles (e.g. sending tiles into empty space, which charges at a faster rate than if they actually hit something). All this while keeping your party’s HP up with your healers or heal item if you could. Final boss stage is where you would unload your Axes/Bombs/Arrows, whatever damaging items you have and your specials. The final bosses are typically mana driven and have nasty side effects to their specials, for Frostmarch, it is defense debuff, so make sure you have either a Cleansing hero on your side or antidotes to immediately offset the effects.
There comes a time when you would make the decision whether you wish to continue with 75 Gems paid up, if it doesn’t look like you have a shot, I would say no. That’s what happened when I couldn’t finish those levels and just declined to continue. If you do not have any 5* heroes needing ascension materials at this point (it will be a while before a typical player would even need to consider non-farmable ascension materials for 5* heroes), I’d even forego this level until you are strong enough rather than exhausting through your battle items and gem stock without much to show for. If you can pass the 4th level, at least you got your ascension materials for your 4* heroes that you would have higher chance of acquiring and build up so you can improve your roster to get pass these stages in the future.