After doing the final battle with Shusui, Bertz comes along and says what Shusui just about said: that the summoner is a candidate for being the strongest being in existence at this time. Now, taken with some context, this could mean something quite significant in terms of enemy strengths, actually. (Do note, this is entirely opinion and is not confirmed fact)

Questing enemies, as stated in the title, are extremely weak most of the time. Typically, it is the summoner who fights them.

GGCs have a theme of featuring units at the pivotal points in their lores. While we go in to them with our own teams, we're actually representing various characters (both major and minor) from those stories (example: for the final battle of the Palmnya GGC, we can be seen as playing the role of Cardes the Malevolent as he strikes them down. There's also the obvious Lucius in Kira's GGC)

In GQs, the enemies tend to be semi-difficult as well. The teams used represent those of the summoner, the summoner's companions, or the other featured characters. The strengths of each team vary, but the Summoner's teams are easily the strongest due to having no restrictions on what can be brought aside from cost.

In trials, the summoner is facing down mock units designed to challenge them and push the limits of their strength. The mock units aren't as strong as the real things (at full power), but Noel, as we all know, is some sort of magician when it comes to mock units, and can make obtaining them a real pain in the rear end.

Taking all of that into account...it seems to me that the strength of the enemies faced isn't really reflected in how strong the enemy actually is when we face them. Rather, it reflects how strong the character the enemies are facing is. Questing is a joke? That's because the summoner is literally the strongest being in existence right now (according to Shusui should the summoner beat Mora) and nothing can seriously beat them when they go all out.

GGCs and GQs giving you trouble? Well, the characters represented by each squad aren't necessarily as strong as the summoner is, so of course they would struggle more with strong enemies. Though when they get to the point where more of their power is released, the final bosses end up being jokes due to the enormous gain in power relative to what the enemy can do (such as Grahdens when he gets serious in his GQ and solos Winged Mashreed no problem, despite normal Mashreed's surprising amount of power for a demon giving normal Grahdens and THREE of the summoner's squads a bit of trouble*).

*Note: at that point in time, the group hadn't traveled to Ishgria, so facing demons of that strength would have been something the summoner would have been unfamiliar with at the time.

Trials really giving you a hard time? Well, Noel specifically designed them with the intent to challenge the summoner and push them to the limit of their power. Of course it would be hard (until the tricks are learned and the loopholes are exploited, that is. Mock units are by no means perfect).

Just something I thought of after hearing Bertz talk a bit about what Shusui said. I think it makes some sense if looked at in that way, though again, this is entirely my own opinion.