Avoid doing sidequests that were given to you by named NPCs, because they net you EXP.

EXP, tech points and skill points you gain from enemies scale to your level. By doing sidequests that reward you with EXP you quickly level up which will make the game easier in the early stages of the game, also because of equipment rewards. But on the flipside you won't amass many tech points and skill points by fighting what gets in your way in the storyline. Thus your abilities and skills may end up being underdeveloped towards the end of the game. Nothing dramatic, it's only going to hit you hard if you want to tackle all the optional content.

There's no harm in accepting all sidequests you come across. The ones you get from unnamed NPCs you can complete right away, because they only award you with money. The ones you get from named NPCs you can also complete in the sense that you slaughter requested monsters or collect materials dropped by monsters. The actual completion only triggers if you speak to the named NPC again.

All of the above is not that important, just keep in mind that the difficulty of the game is largely in your own hands. If you want to be overpowered, do all sidequests as soon as possible. If you want more challenge, stay clear of sidequests.

One aspect that is important is the affinity between your party members which allows you to link more skills (very important) and also raises the probability of longer chain attacks. During battles, you are sometimes prompted to press B when one character dodges an enemy attack or something. Hitting it perfectly when for example Reyn dodges in a party with Shulk and Sharla will net you three hearts between Reyn and Shulk and three hearts between Reyn and Sharla. Pressing B a little too late reduces these values to one heart. Activating a chain attack with the same party will reward you with one heart between Shulk and Reyn, Shulk and Sharla, Reyn and Sharla. For this reason I recommend to activate chain attacks even it is completely unnecessary (like right before the end of a battle you would win anyway). This stuff will add up over time.

Also, when you accept and complete sidequests from named NPCs, your active party will talk to each other and the leader will always be among the two characters that receive hearts. So if you have a character that lags behind in affinity, put him/her in the lead of your party when talking to named NPCs.

I just defeated Mumkhar for good at Galahad Fortress and I was at level 52 when I defeated that particular boss, do anybody know if I was over- or underlevled and I can't find the next checkpoint for some reason, after the boss. 

How do i measure affinity? Haven't quite figured out all the menu functions yet...

Go to the affinity chart in the notepad menu button (it looks like a group of people in a circle with lines coming out of it, then click the central icon in that menu).

Other things you shouldn't forget:

-You can fast travel to any area in the game by going on the far-right icon

-You can gift collectibles (the blue orbs) by going into the inventory and selecting the item

-If you press the b button during burst affinity right over the outer circle, you get 3 hearts increase, not 1.

-Quests from unnamed NPC's give no exp., and quests later in quest chains give better rewards (better weapons, more affinity stars, skill trees, Monado arts etc.)

-Talk to all named NPC's in a new area once, then return at a different time (e.g. 3 hours later) and talk to all NPC's twice, to gain access to more affinity links and quests (I did this in Frontier Village and gained 11 quests, when before there were none!)

-You cn buy intermediate art books in shops.

I think that should just about do it. You'd be surprised how many people don't realise especially the top 2, andonly realise halfway through the game >.<

Where abouts are you in the game BTW? I think early on, but that was a couple days ago.

The Bionis' knee. Haven't been playing as long as i should be...

Each character has their own kind of weapons, so there's no point in saving old weapons. I think you can trade them, but I never did. You will get the best weapons from unique monsters and quest rewards, but never from trade. So sell old weapons and invest the money in art books.

Honestly, you never have to buy any weapons or armor. Drops from monsters and quest rewards are sufficient and usually better than what you can buy anyway.