Balancing With Tactics

Focus Fire. Concentrating attacks on one or two heroes makes encounters feel harder. Conversely, spreading out damage or sending monsters in waves can make the encounter more manageable.
Movement. Monsters that move each turn force martial characters to reposition, limiting their attacks. This also opens up opportunities for reactions like Opportunity Attacks. To lower difficulty, reduce monster movement.
Target Squishy Characters. Make them use their actions to defend or run away and hide behind their stronger friends! Let the tanks feel good about Defending and Interposing.
Adjust Initiative. The sooner the monsters act in the round, the harder the encounter will be, and vice versa. If, after the first player’s turn, it looks like the bad guys are in for a trouncing, move some of them up in the initiative order instead of waiting until the end of the round.
Relax! Heroes have plenty of tools to handle tough situations, so don’t stress over getting every encounter perfectly balanced. You’ll improve with experience, as will your players.
Hero Death Isn’t the End. If some (or all) heroes die, it’s not necessarily a failure. Encourage your group to make new characters and dive back in.

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