Old Deadly Red Spiders are good f2p monsters to train on as they had low offense, defense, and were afkable.
The new Deadly Red Spiders were overbuffed compared to the pre EoC versions and they have decent defense now.
Currently they are level 136, the highest level for a normal monster in f2p (WTF). They are also not aggressive, so you can't afk them.
F2P combat training got nerfed hard due to the drastic changes from the EoC. For high levels, the only afkable monsters are in the wilderness, but I don't think you want to be afking in the wilderness.
The new wilderness did give me a warning that skulls were removed and you lose everything if you die, so there goes the 3/4 iteming afk grinding method. I haven't tested out if the protect item prayer is disabled though, so 1 iteming might be possible.