When my students have trouble with the red words I break them down into the Phonetic ones and teach those first (those that sound out normally). The red list is huge with a lot of words that break regular spelling patterns. It's important that when teaching these words you do use any phonetic links and point out when they are not completely phonetic. Then, add the harder ones 1-2 at a time (2 when they link like we and he and teach your child that these words use the long/other vowel sound). Teach/expose your child to the word or to build towards "for".
There are some great activity suggestions here! Others I like are posting words over doors and entry to rooms is via that word "password" -either spelling or reading depending on focus. Old school cut and paste or magnetic letters works really well for those words that share spelling patterns (or/for, as/at). Sensory activities tend to engage kids the most! Oh and writing words on large duplo and then the letters on smaller duplo pieces to match and make.