As a big time fan of tower defense games I have modified many rules to this game to increase variability and make each session as fun as possible. One of which was to add swarm type to the wave selection.
There are many ways you can achieve this, but the easiest way I found was to do as follows:
I will use wave 1 for an example. It says to summon 3 monsters. Now rule the D12, now if you role:
1-5 it says summon a level 1 monster. So you summon a level 1 monster.
6-8 it says summon a level 2 monster. So you summon 2 level 1 monsters.
9+ it says summon a level 3 monster. So you summon 3 level 1 monsters.
So depending on how you rolled you could be fighting any where from 3-9 level 1 monsters. You can also increase the difficulty by using any level monster you like. Due note that using this idea will not work on all waves. For example if you are on wave 3. You could summon any where between 7 - 28 level 1 monsters. Which will end up with more monsters on the board then you will know what to do with. As well as running out of monster tokens. I came up with a few solutions to this.
Instead of only summoning level 1 monsters. You summon 1 and 2 based on what's rolled. If you rolled a 4 on wave 3 which says summon a level 2 monster. Instead based on our rules you should summon 2 level 1 monsters. Now sense we don't want to run out of them and we should instead summon a level 2 monster and a level 1. Now if you roll a 7 which says summon a level 3. You summon two level 1 and a level 2.
A second option is to summon level monsters in this manor:
level 1 summon a lvl 1 monster
level 2 summon 2 lvl 1 monsters or a lvl 2 monster (flip a coin?)
level 3 summon 3 lvl 1 monsters or a lvl 2 monster and a lvl 1.
level 4 summon 4 lvl 1 or 2 lvl 2 or a lvl 2 and 2 lvl 1.
level 5 summon 5 lvl 1 or 2 lvl 2 and a lvl 1 or a lvl 2 and 3 lvl 1.
Obviously you can always use the same rule and summon them in the same manor, is use a coin or dice roll to determine how that swarm should be summoned.
A third option is to summon them in groups of 2-4. Wear you roll and summon the swarm wave and have them move and attack. The allies get a turn, then you summon 2-4 more just as if you were playing gauntlet mode, but only summon the required needed for that wave. This also helps keep the numbers down so you can re use the monster tokens because you will run out of the with this mode.
I never tried playing the entire game like this, I only did this to mix up the waves and would only do it once or twice in a 5 wave game.