He thinks Strickland is likely to ridicule him. That means Strickland will mock him or make fun of him. Strickland is likely to hurt his feelings. So he doesn't want to seek Strickland out.
Before seeing Strickland, he would need to "steel himself." That means he would need to prepare himself emotionally for Strickland's verbal attacks. He would need to protect himself, as if wearing steel armor. If he wanted badly to see Strickland--if he had "a very lively passion" to see Strickland--he would do it.
A "pose" is pretending to have higher motives than you really have. For example, pretending that you want to help the poor, when what you really want is to feel superior to them. Strickland is good at seeing through poses. He likes to puncture them. He is sardonic: he says nasty things in a clever, witty, or humorous way. But Strickland has a sardonic sincerity. He really means the nasty things, so he his attacks really hurt.