See, this is the same kind of issue you can have with Power Rangers.
1) My 7 year old summed it up best, "Dude, change already." I mean, okay fighting without your powers.. very dramatic.. but not when the Fate OF The CITY is on the line. They get lucky that the GM is nice to them and that they don't get KO'd in that first set of exchanges.
2) Why aren't more buildings flattened? Not every street is any city I have ever visited, is wide enough for a proportionally 10-15 story humanoid. Certainly not the side streets.
3) Why the same city? If I was a villian, I would launch some of the mosters in another city, buying me some time while the heroes travel there or burn their energy teleporting. Heck, most heroes could not find your plot until they stumble over it (or their command team bothers to check their scanners). Send the plot somewhere else, stop trying to take over the worldy by takinon Angel Grove first and establish a beach head in Omaha
4) You know their Secret IDs. You know when they are sleeping. You say you are Evil. Bombs? Jumping them when they are asleep and can"t quite get to their morphers in time? Or you can be more subtle. Food Poisoning in their parent's kitchen, heck just something toxic in their tooth paste.
5) Thre are only five of them, (most of the season), how hard would it be to unleash six monsters or lts out there at once. One could "win" for a time. Eentually thy will get smart and bring the team togteher aginst on of the threats a a time, but how much damage can you do to the world while they take them down one by one?
6) Never make the monster's big. The megazord is much more powerful than the clowns are in their spandex mode. Either start with the bigger monster or never make them huge.
7) Stop making giant monsters, create monsters that look like people , good looking people, in suits. Unleash them in the city, but make sure their powers are never visible. Make sure the monsters are aroudn people and crowds. Watch the power ranger's lose the PR battle as they take out "the helpless civilian".
They are teenagers, how hard are they to distract? If they show up late to the battle, that is one edge you have. Make sure the attack is on prom night. Arange for them to have hot dates. Arrange for them to be on the football team or somewhere with commitments. They have to be there for this audition or they will eve get the record deal. Maket this happen.
9) Nuke the Command Center. You might not take it out, but you will put a cramp in their style.
10) Parents, blackmail, do the math dude.