Iron Snout

A combo in this game just means consecutive hits on enemies without taking damage. You can wait between hits as long as you want. All that matters is that you don't get hit. Each enemy takes around 5-6 hits to kill him, so you're likely to reach a 25 hit combo within the opening few seconds or minute of any game mode. "Classic" mode is a slower flow of enemies, so it may be more conducive to racking up a high combo, but you'll be doing enough grinding in "Pig vs 100" mode that this will almost assuredly happen naturally. To put it in perspective, my average max combo in a game was in the 50s or 60s, and my best was 98.