so it results in the fastest dwarf getting surrounded and killed before the rest reinforce him.
There are several factors that can come into play. Dwarves report for duty when they are ready, after collecting equipment. If you issue a "move" order to a safe staging area, your dwarves will trundle in at their own pace; once assembled, you can 'move' them out as a group. Avoid using long-distance 'move' orders--that just increases the disparity between the slowest and fastest dwarves. Instead, order your dwarves to 'move' close to the battlefield, then order them to 'move' into combat. Don't use 'kill' orders; once the target dies, the 'kill' order is automatically canceled, and any dwarves who haven't yet reached the battle will wander off to do something else. (You can issue 'kill' orders for multiple targets, but there will be some delay as targets are recalculated.)
In general, a little disparity is good. Dwarves with high armor-user skill will reach the battle first, drawing enemy attacks before the less-skilled dwarves arrive. This allows the trainees to get in a few attacks before they're knocked senseless. But if your trainees are very slow to arrive, it's unlikely they have enough skill to influence the battle at all. A skilled military dwarf can frequently survive 10:1 odds (and worse, for a limited time), but there are random factors that make combat unpredictable.